My error = We proofread in Microsoft Word (well, actually iWorks Pages) but then...
During my later editing for layout in Adobe Indesign (the program we use for Yearbook print layout), the difference between keying "Esc t" (for add a Type window) and just a plain ol' "t" ended with a bit of humor.
As I was trying to place a new type box, I added a little ol' plain "t" and planted a "t" right there in the sentence. Because I was moving too quickly, and without noticing, I made "newt" instead of what was there .... "new". This error actually went to print but it was a newsletter for friends and luckily not for Yearbook.
Of course any dumb ol' spellchecker accepts "newt"* and I did not see it.
So there you go...duh. One reader caught the error and brought it to my attention, writing that he was please to know that I had made a "newt" friend. It was a great dumb ol' professor error for my show-and-tell discussion when returning to Yearbook class the next morning. Most of the students did not catch the humor.
I did appreciate it..however.
*for those of us unfamiliar ...a newt is an aquatic amphibian of the family Salamandridae. (wiki)