I wouldn't make a habit out of it, but the rare occurance won't hurt anything. It's mostly just a matter slightly increased wear. this isn't a true high performance motor, so you're not going to permenant damage like warping the heads or cracking the block... as could happen in something like a 1000HP Formula 1 engine.

Sorry but you motor will only last 398,000 miles now instead of 400,000 miles.