Can't transfer your skills to the shiny new thing.

fighting the urge to rewrite everything and propagating the legacy code cycle.

Hands down, the urge of product to assume that codebase is stable and as nimble as a greenfield project, and so any feature can be build with minimal “we just need to make a small change in feature X to do Y”, while never allowing for resources to allow the legacy codebase to keep pace with new features.

The tiniest feature becomes a chess game with unpredictable interactions, not enough tests, etc.

Legacy expectations and requirements. It's not uncommon to see that there is a better way to do something, but be unable to do it because the customer assumes and/or expects the current way of doing things. They might even be on board with the new way, but trying to include that in any sort of project budget is a non- starter.