Old, and/or outdated documentation is my top struggle. With all these different versions of .net framework/core, asp.net with and without core, entity framework and all the other stuff in the ecosystem you always have to check if the documentation fits the .net (framework/core) version you are currently using.

On the first look it seems like there didn't change a lot between the old net framework and net core 1,2,3 and now .net 5 but once you get into the details each time it is a changed world.