I was working with a particular spreadsheet editing some formulas. The spreadsheets I work with always have the panes frozen which, if you didn't know, can make it look like you're at the top of the sheet when in fact you are NOT. The sheet had dates on the left side, and the one at the top was January 2010. I didn't think much of it even though EVERY OTHER TAB in this file started with January 2009. I just went through editing and copying the formulas down the column. Only at some point I scrolled all the way to the top and then when I looked back through the formulas I had edited they were all wrong! I could not for the life of me figure out what had happened. I just changed all these! Why are they not changed?! And I freaked out a little wondering what in the world happened to all the work I'd just done. Then I figured it out and felt like a giant idiot.
The mistake was easily fixable, but I really thought I might be crazy there for a minute when I couldn't find any of my work. I was definitely awake after that though.