This was actually informative. I just started exercising again and I'm doing two of these in my routine. Doh!
Yeah, the behind the back shoulder excercises are the worst offenders. Back when I first went to a gym, I actually had a trainer recommend behind the back lat pulldowns. Anything that strains your muscles into an unnatural position is a big no no.
Some trainers would consider this heresy, but honestly I would not recommend using weights at all. There are plenty of bodyweight only excercises that get the job done. I like to think I am reasonably fit and am proud to say I haven't used weights in over a year now. Feeling pretty damn strong for it too. I could steer you towards the resources I learned from if you'd like.
Example: See that thing about bicycle kicks with your hands behind your head? Do the same thing, but while you kick, do the same motion with your hands, like you're plucking things out of the air. That will intensify it even further since it engages your upper body and your arms aren't just passively hanging out around your head. You'll also engagle stabilizing muscles in your back since you're now balancing on your glutes without larger arm muscle helping the upper body stabilization.
When you work up to handstand pushups, you are playing with power! :)
RE: Excercises you shouldn't do.