Numerous times. Early on last year, when I first got the game, I made a Japanese friend via island tours. She gave me some gold roses, tools, perfect fruit baskets, and hybrid purple flowers of various kinds as a gift. When I ran out of storage space, I ended up leaving some of those things out on the ground by my home and the flowers were placed outside of Town Hall (except the gold roses, which I planted next to my house). 
Feeling good about making new friends, I took my search to tumblr, where I was very active. I found a few people to exchange codes with who said they would like to come play island games. Out of the four that I invited, three of them stole flowers/fruit and tools from me that I had been given as a gift. I caught two in the act and they ran for the station. At the time and being a complete newbie to the game, I had no idea that you could force an error. I lost everything but two gold flowers out of the bunch. After that, I've been very careful about who to add, but even that can't protect you at all given that others can be perfectly friendly at first even if they plan to swindle you.
I'm very grateful for the ability to bring the hammer down on them now with a forced power-down. It's probably not too good for the 3DS, but it at least protects your town from any damage/lost items. I wish I had known that before I got my treasured gifts stolen.