After I successfully adjusted the time on my Speedy (despite Archer's suggestion that those of us who don't know what we're doing should keep our grubby little hands out of there!) I decided to do the same on a cheap Chinese diver (not a replica, that would be against the rules here). It was something stupid like 20 minutes fast. So, I popped off the back and adjusted the doohickey thing. Better. I adjusted it some more. Worse.
Ooops, so I fiddled with it a bit more until I finally succeeded in popping out the entire balance wheel assembly, destroying the spring in the process. It went on eBay at a substantial discount "as is".
I learned my lesson, and realize (now) how lucky I was with the Speedmaster. Although, to be honest, I took a lot more care with a $7K watch than a $200 watch.