Weird Extrude Face Snapping
 

I am at about the 33 min mark on the Apache tutorial, module 5, and I've encountered some weirdness when I try to extrude. Up until now, aside from some weirdness with manipulator direction, the extrude face has been working like a charm. However, as I was trying to extrude faces on the front of the Apache, all of a sudden the lower left corner started to "snap" to an area behind the smoothed mesh.:headbang:

I've attached a pic that has a before and after pic, as well as my tool settings and my extrude face tool settings. I cannot understand what is going on!:confused:

The settings are the same has they have been all along and I cannot figure out why it is acting this way. I know it must be something simple, but I don't know what it is. Please help!!!!

Thanks.