Probably every single one you can imagine.
But it doesnʼt really matter. I donʼt even remember most of them, just the fact that I failed badly a lot of times. What matters is you donʼt do the same mistake twice unknowingly. Or thrice, tops.
For the first time, especially if you donʼt have a mentor, you will realise the mistake too late (like, after release.) If the second time happens, you will realise it early enough the thing doesnʼt go to production. And the third time should never happen. Well, unless…
Thereʼs an old saying, “I can do it well, fast, or cheap. Choose two.” You simply canʼt do all of them, and most of the time it will be “fast and cheap”. In those cases you will knowingly do mistakes on the altar of fastness. But this time you can add a comment like:
# FIXME: I know this is wrong, but I had to do it fast.
Learn from your mistakes, and donʼt repeat them unless really necessary. Do so, and your mistakes wonʼt even be remembered.