One morning at home I picked up from the coat rack my DS1's school fleece, to hurry him up, and was appalled to see it was clearly labelled "Girl in DS's class."
His own fleece was also on the coat rack. He had obviously considered it a bit nippy at school one day and decided to put his fleece on. Except he had not worn one that day so he had picked up any old one.
I had no idea how long we had had it - it could have been a week! I was horrified; luckily I knew the girl's mum so returned it with apologies immediately. Luckily she hadn't really missed it yet, she was just starting to wonder where it was.
I can assure you that there was no intention to steal on either the part of my DS or me.
Similarly, I noticed one day that my DS's shoes were not his. They were the same style but neither one was his and one was two sizes too small. I tracked down one easily enough. The other one was more difficult, DS of course being airily clueless about who he was next to when he changed for PE last. Then I noticed another of DS's friends in the playground at hometime, same shoes but one clearly much bigger than the other. I approached him cheerily and said "X, I think you might have got one of DS's shoes on." He was adamant that he had not. I had to be a little firm, that we check, and sure enough, one of his shoes was DS's - 2 sizes bigger.
When I read threads like these I sometimes wonder whether I should have accused this child of trying to steal one of DS's shoes because one of his feet is bigger than the other or something . . . 
I get so frustrated when I read these threads, it's as if no one will admit the possibility that their child loses things or doesn't bother to look at name tapes, or doesn't look in lost property properly, or doesn't go to lost property at at all and lies about it, OH no, it's always some criminal other child/family trying to steal your school uniform,.