The bolt holding my front derailleur came off, (didn't notice it), and when I shifted, the whole thing came off, and the chain locked everything up. Fortunately, I was only 2 miles from work at the time. I had to walk the bike up the hills, paddle foot on the flats, and could coast down hill.

Walked the bike over to the local shop when it opened (about 5 blocks away), and they fixed it in ten minutes.