Every now and then when playing TF2, I see a spy who seems to be able to stand right in the middle of a small group of enemies, whether in disguise or in stealth, and doesn't seem to have any problems staying with the crowd and not getting bumped.
However, it seems that when I (and the majority of spy players) are in a crowd of enemies in disguise, someone bumps into them and they are discovered. What are some good tactics for judging when a player is going to back up or suddenly turn towards you? What's a good way to avoid bumping into them without making obviously suspicious movements?
The moments I am most interested in are those times when you aren't ready to backstab anyone, and you want to wait until the group gets into a firefight so that you can more easily pick them off and escape... I don't seem to have too much problem moving in for a kill, just staying with the group without being bumped.