Don't know of any place where that could happen today.
You'd be surprised. Many ops don't set up their games properly. With easy settings, a dollar or two can last for hours. Stern default replay scores are generally low. I think the first default replay score for Spidey was only about 19M. Pathetically low. Add in extra balls, specials and credits awarded for beating high scores and you can collect credits in a hurry if you're a halfway decent player. Reflexing, or auto percentaging can help, but if the replay score starts low to begin with, doubling or tripling the floor replay score is often still easy.
I give away credits on location whenever possible. Now that I'm older though, it's getting a little tougher. Younger kids will take free games in a heartbeat. Not shy at all. Their parents have told them not to accept things from strangers, but it hasn't really sunk in yet. Besides, I'm not giving them the whole game, just a few credits. Teenagers are tough. They're more wary and are too busy trying to look cool to take free games from some adult they don't know. You can still give them free games, but you have to do it subtly. Sometimes just a look and a nod in the direction of the game on the way out will get the message across. The kid, trying as hard as he can to look cool, won't run over to the game right away. But as soon as you get out of sight, he's all over it.
If there's no kids around, I'll sometimes burn all the extra credits except one, depending on who the operator is. Good op, burn all but one. Bad op, leave them all. Always leave one though. Pay it forward, or something like that.