Let's start with the biggest source of hidden problems—manipulating a user interface from a background thread. This is, surprisingly, quite a common problem—even more so as all Delphi resources on multithreaded programming will simply say to never do that. Still, it doesn't seem to touch some programmers, and they will always try to find an excuse to manipulate a user interface from a background thread.
Indeed, there may be a situation where VCL or FireMonkey may be manipulated from a background thread, but you'll be treading on thin ice if you do that. Even if your code works with the current Delphi, nobody can guarantee that changes in graphical libraries introduced in future Delphis won't break ...
