All the time. Drives me nuts. It usually means that I'm done with that toon for the day, since if I leave the writ turn-in area, I'm going to get nabbed by a guard, and I refuse to pay 92 gold over some stupid sword that I didn't want in the first place.
I've only done it in Riften. Once it was a green staff. With the quickly forgotten and kickback passives, I actually made gold off the transaction by the time I walked around the lower level and climbed to the Outlaw's Refuge. When it was a white staff I just logged out and went on to the next character.
Every so often. Vivec City during the anniversary was a bit annoying. Between rushing to do many alts of writs, and the massive crowds around the turn-in (all those pesky bears and winged twilights!), I was lucky I only did it once. Of course, they have a guard posted right there.
Ive done it a couple of times but not by looting containers. Its that damn weapon STOOD UP AGAINST A WRIT BOX in Elden Root. Who's bright idea was it to do that?" Oh guys lets make this look real and stand this weapon up against one of the writ boxes, but make it so it flags stolen. Its not like a few hundred people a day arent going to be clicking on the box RIGHT BEHIND IT or anything"