Bike is an '06 Schwinn S-25 with Shimono gearset. Chain fell off the innermost sprocket at the rear wheel and became quite stuck between sprocket and frame. I was able to pull over and free it using the wheel quick release skewer.
It happened while I was putting power down while shifting from 5th to 7th.
I was really enjoying the transmission on this bike vs. the grip-shift on my previous ride, until this happened.
So I'm left with one of two conclusions: either I can't pedal hard while shifting, or I have to adjust my rear derailleur (after studying French spelling, of course). Which is it?