I have a few.
One is abbreviation hell. Nobody is going to spend the time trying to decipher what you mean when you use over several abbreviations. It is just better if you’d explain than expecting people to understand aside from commonly used abbreviations that are easy to understand.
Another is overstepping your limits for the sake of getting a partner. Compromising your own standards is perhaps one of the worst things you can do when it comes to trying to find dates. Like you’re suddenly okay with dating single parents but you don’t like children. You’re suddenly okay with dating religious people but you’re not religious. Things like that. Because it means you’re desperate and you’re setting yourself up and setting them up for a bad date.
Not sure whether it’s a mistake or not, but when I see people being so negative on their profile, I skip. It’s fine to announce your intentions (eg, looking forward a commitment, not looking for a situationship), but if your profile is mainly a list of don’t’s, then that to me screams you’ve still got issues to work through.
Another one are the people who either don’t fill out a profile, or say something trite like “if you want to know, ask”. That makes me think you’re lazy, and expect the other person to do all the work. If you can’t be bothered even a little, then I can’t be bothered even a little.