Well.... mistakes. We all made them. A lot of times you the root of it is communication. Better said: a lack of it or misunderstandings. Let's see what mistakes I remember ;)
World building: I never was tempted by that. I prefer to take a setting and run amok. What can you do while still staying true to the lore - that's my kind of thing.
Info dump: well... coughs ...you know... Partly guilty. Sometimes I asked the players "Should I send you a mail with the background information and you read it till the next session or do you want to learn that in-game?" It seems players are a lazy bunch and prefer the latter. So I spend half an hour of precious gaming time to have different NPCs tell the group what they need to know. Of course there comes the question "Do you have a handout with the stuff"? Of course I have and you could have read it already....
And my foible are descriptions. I picture a place or a person and try to include more than just vision, talk about hearing and smells and tend to make it a bit too long. At least that's what I often hear after a session.
Railroading: Well, I try not to. Unless it's needed. Depending on the group you can tell them "it would be great if all of you go + visit the druids*. But this thing with preventing that they find clues too fast (or, more specific, before the found a more important clue) ... well... there might have been times when a certain NPC just wasn't at home ;)
Memorizing everything: Nope. I read an adventure, of course (information about NPCs and places tend to be where you last expect them) but make myself a rough schedule for a session (where are the PCs, what did they plan to do, what information should they receive etc., what is happening elsewhere plot-wise). I can adjust that after a session.
And why do I do that? Well, there might have been an adventure (detection in a roughly Amsterdam-like city where important people drowned - accident or not?) where I forgot a death (OK, it was badly written). But the whole session the players just couldn't understand why things happened and what the clues meant. That was a long email full of retconning when I realized my mistake...
Hand-holding I might have sent a player or two messages "you suddenly remember..." with hints because I want the story to go on. A riddle is fine as long as it doesn't get ridicuous ;)
Regarding player knowledge vs PC knowledge: I'm quite lenient in the beginning - I tend to disregard rules if they are not needed anyway. But if a player doesn't know after 20 sessions what a feat / spell etc. does - sorry, if you don't know it you don't have it. It's so easy today to make an info sheet for a PC that there are no excuses.
BTW: is there an English term for disregarding/adjusting rules because they're not important at that moment? In German we use "hand waving".
Hand waving is also how we refer to it. 😁
This, right here, is the kind of comment I wish I could upvote, like, ten times instead of just once. Thanks so much, @muscara! And yes, as @jamethiel said, the English term is also "Hand-waving". Funny how languages overlap like that, isn't it? :)
It might be that the German term comes from the English...