okay this just happened to me today, and it was very nerve-wrecking, and easily avoiadble:
I finished my thank you letter for a position I really want, and I went to the nearest post office on my lunchbreak to mail it. After I mailed it, I was driving to the nearest fast food joint to grab some lunch, and realized I might have made a mistake. while I was proofreading the letter (and then signed it), something creeped in the back of my mind that I forgot to actually type my name. In its place was the "Your printed name here" that was orignally there. This is bad for two main reasons:
1. It looks unprofessional
2. Its the only legible part on the letter that signified WHO actually wrote it

so, for about 10 minutes I cursed about everything I could think of under the sun and started to get REALLY worried. I cant remember when my stomach felt like it did. I even checked my letter in the line of the post office, but STILL couldnt remember if I printed my name or not! So the worry wart that I am, I got my lunch ran in the office and checked my email (I sent myself 2 copies just in case) and sure enough my name was there!!! WHEW, talk about a brain lapse!!!!

moral of the story: PROOF READ PROOF READ PROOF READ!!!! Triple check your work if you have to. I normally do, but for some reason I rushed or didnt check my work as thoroughlly as I normally do (at least twice). Have somebody else proof read your work if you can.