I've been using Blender for two days now, and ran into an issue while following a tutorial.
Before anything else, I'd like to show you all what my problem precisely is. Here is a short video I uploaded: click here. I thought it may be easier for you all to help me identify and solve my problem more accurately with a visual aid.
I would like to emphasize that this issue began to occur after I extruded that trapezoidal region (misstated as a parallelogram in the video). I decided that I did not want the extruded region, and just hit undo. Ever since then, I began to experience the problem you viewed.