During land battles, did soldiers get confused about who was on their side and who they were fighting in the melee? If not, why not?

There are a fair lot of methods as to how to do it, depending on location and time period. We can say that the screen depiction of a confused mass of all sides intermingled with each other is very much Not It. Here's a few previous threads on the matter:

u/400-Rabbits looks at how the Mesoamericans did it;
u/WARitter examines how they did it in the Medieval Era;
but this, clearly the best answer, is one straight from His Majesty u/Henry_V_Rex.