I know exactly what Bob means on this one - it's one of my favourite pet peeves. Approaching a red light, with a car in the left lane ahead of you, but apparently intending to drive straight through when the light changes, since they're not signalling a left turn. So you pull up behind them at the light, and when the light turns green ... then they hit the turn indicator. But now you're stuck behind them for however long it takes for them to be able to make the turn. If they'd had their turn indicator on while they were waiting, you'd have known they were turning left, and you could have changed into the right lane and not got stuck behind them.
Basically, it comes from not understanding the purpose of turn indicators. They aren't for telling other drivers what you're doing at this moment: they're for telling other drivers what you're planning on doing, giving everyone a little advance notice of your intentions
Check it, Bob, we got another one on the line..