Mysterious iPhone data usage while on Wi-Fi

I received a text from my cellphone carrier the other day warning me I'd used 90% of the data on my plan and still had 10 days to go before the end of the billing cycle. I thought that was strange, so I logged into my account on the carrier's site and it showed a usage of about 414MB the night before. A call to tech support confirmed the same. I only have a 500MB plan and therefore am usually pretty careful about mobile data usage (no downloading or audio/video streaming). The weirdest part about all this is that I was at home and connected to Wi-Fi the night in question, and I hadn't even used my iPhone all that much anyway. I remember checking Twitter a few times and that's it. Even if I had lost my Wi-Fi connection, which I would've noticed, there's nothing I did that should've used anywhere close to 414MB.

Since this happened I went to Settings>General and switched Mobile Data off, but now I'm afraid that when I turn it back on the same thing will happen.

Does anyone have any idea how all this data got eaten up? It's my understanding that, for iPhone 4 at least, all data should go through Wi-Fi when connected.