For instance, I expect older people to be less familiar with technology, or I expect that very young children can't read, or I expect that the guy trying to get my phone number at the bus stop is not asking because he thinks we have common interests. All of these assumptions can bite you in the butt if you act on them, and sometimes you can learn to avoid communicating potentially offensive assumptions you have about people, but it's pretty common for those preconceived ideas to be revealed, if subtly, through interaction in ways you may not be aware of.
This is one of the reasons why, when I discuss the importance of supporting asexual friends and family members, I stress education. Because one of the default assumptions MANY people have about asexuality is that it is a phase that will pass, and even if someone who believes that is kind enough to avoid saying so to their ace loved one, that belief will often come through unintentionally in their interactions. We can hear in your voice and your choices of words that you expect us to grow out of it or that you don't believe us. It isn't particularly hard to tell when you're being humored.
Here's another example: an interaction between me and a friend. I had discussed a health problem I sometimes experience, and he immediately started asking me weird leading questions about it. It went something like this:
Him: Did you start noticing that happening at any particular time?
Him: I mean, like, did you make any . . . lifestyle changes around any particular time in your life that might be related to this?
Him: Did you ever think maybe it might've started happening . . . around the time that you became a vegetarian?
Oh, I saw where that was going long before he said it.
I've known for a long time that he thinks vegetarianism is unhealthy and that humans generally "need" animal protein to be healthy. He has said to me before that he thinks vegans "look sick" and that he believes our bodies depend on animal nutrients, and also he has said before that he believes vegetarians and vegans tend to be stuck up and obnoxious about their supposed moral superiority. (I don't know if that includes me.)
So, I've been a vegetarian since I turned twenty. That's seventeen years. My doctor's only comment on the situation has been to say that I should take a calcium supplement because in addition to being vegetarian I am lactose intolerant so I don't get much dairy calcium. He has said nothing about vegetarianism causing the other health problem (which, incidentally, is a very common skin infection that I very likely am more subject to because I ride a bicycle in the heat and then stay in clothes I exercised in). But if you have a preconceived notion that vegetarianism is by default unhealthy, you may be more likely to erroneously blame unrelated problems on it (and ask sort of obnoxious leading questions to annoy your friends).
It gets worse when, say, healthcare professionals allow their very human biases to affect their judgment. It's been studied and found many times over that doctors and other healthcare workers sometimes have alarming beliefs about how one's race affects their pain tolerance and whether a woman's description of her symptoms is accurate. Men in our society, by and large, assume women are overreacting or exaggerating, assigning us "emotional" perspectives that predispose us to illogical evaluations of our own experiences. This translates, in practical terms, to their inherent distrust of women, and their conscious or unconscious tendency to downgrade the seriousness of anything they say, including descriptions of their health problems. I saw one man quoted in an article opining that "women" are just far more emotional than is "logically" warranted, and so when his wife is on 8, he assumes the situation is a 5 or a 6. This perspective has also led many healthcare workers to ignore women's medical symptoms and suggest they are simply excitable, worrying for nothing, overreacting, and yeah, probably just suffering from anxiety.
Going into conversations with people carrying preconceived ideas about their situations can sometimes be helpful, of course. This is why it's an adaptive strategy that humans evolved to have, I imagine . . . it is often useful to our survival if we can make predictions based on snap judgments. We may assess a situation as dangerous in an instant based on a snap judgment, and we may be RIGHT, and if we're wrong about its being a threat, we still survive. It's better, evolutionarily, to regard something that isn't a threat as a threat than it is to assume something that IS a threat will not harm you. We're a suspicious species, with speed of decision being very important to continued existence. But sadly this does not help us make more reasoned decisions rooted in empathy, education, or evaluation of evidence.
As an experiment, a person can make a list of five judgments they tend to make quickly. Maybe you can't think of any at the moment, or they're all really obvious things, so take a day or two to catch yourself making quick decisions based on very little information. Why do you pick a certain grocery line? How do you immediately feel about the car in front of you based on their bumper stickers? When you hear an author's name and you know their demographics, what do you expect to see in their book? If you hear someone watches a certain TV show or listens to a certain radio station, what do you believe you know about them? What about if they shop at a certain store, wear certain clothing labels, or go to a certain school? What do you believe you know about someone if they say they're from the Baby Boomer generation, or they're a Millennial? What makes you think better or worse in an instant about a parent based on their treatment of their children in front of you? What judgments do you make if you know someone posts on Reddit, or posts on Tumblr? If someone subscribes to a certain diet, how does that affect your opinion of them?
The big question is whether these evaluations lead you to trust someone. From the beginning, knowingly or not, people you meet are taking action that either solidifies or contradicts your initial assumptions. How many times might a racist's beliefs about people of color have to be proven wrong before the racist modifies their beliefs? How many times might a homophobe need to meet a gay person who isn't stereotypical before they decide the stereotypes are misleading and reevaluate why they believe them? How many times might a sexist man have to be outperformed by a woman before he stops believing women aren't as good at the job? How many times can you make a reasoning mistake you didn't realize you were susceptible to before you realize you're considering information that doesn't apply? And how often is your trust or mistrust dangerous--to you and to them?
You have to acknowledge that most of these assumptions serve a purpose for you. They make you feel superior to someone, or make you think you're good at something, or make you believe you're being safe or logical because you allow these generalizations to affect your decisions. But think about the times someone else's assumptions about you have inconvenienced you, annoyed you, or even endangered you. Did someone give you terrible service in a restaurant because they have a racist belief that people of your ethnic background don't tip? Did someone assume a person with your gender identity is out to victimize them? Did someone decide to give an opportunity to someone else because they erroneously assumed you weren't physically or mentally capable of completing it?
And sometimes it's just ignorance, like someone has decided gluten-free food trends are annoying and unnecessary so they ignore someone's request for a gluten-free meal and end up harming them because they have celiac disease. Or someone has decided a black person going into a store open carrying in an open-carry state is somehow more likely to be out to commit a crime than a white person would be if they were doing the same thing, and they call the police and violate that person's rights. Or someone in medicine decides an overweight person's health issues are all a consequence of being fat and refuses to investigate a very serious health problem, instead prescribing weight loss until the person narrowly survives a medical emergency and finally gets a diagnosis.
It's important to become aware of your preconceived notions so you can evaluate whether they're useful to you, and more importantly, whether they're harmful to someone else. The tendency to jump to conclusions and act on prejudice may have been a useful tool of survival in the past, but that is less true now. The best way to educate yourself on these issues is to listen to the voices of those writing about their own personal experiences--like I am with asexuality activism and feminism--and understand how common certain prejudices are and how they affect marginalized populations. You can learn to be part of the solution if you'll do this, but it starts with recognizing that the preconceptions are happening in the first place.