One common pattern in this system was that you'd first save the current environment, run the special "environment preparation" function, then the real function, and then write back the saved environment over the modified one, so that you didn't leave the environment changed after you returned. Unless of course you meant to change it. This was sometimes documented, you'd write in which globals a function expected and which it modified into a docstring.

This was probably only in the top ten of the problems that this thing had, but I do remember it vividly. Making any change was like pulling out a Jenga block and replacing it without toppling the tower.