Found this notation in my manuscript, and because I hate to ask stupid questions (a serious character flaw), I looked it up online instead.
Here's what I found at Buried in the Slush Pile:
Stet - Latin for let it stand. Editors and proofreaders place the word stet in the margin of a manuscript to indicate that a marked change or deletion should be ignored, and the copy typeset in its original form.
Well, hallelujah! And here I was thinking it was something really bad...
"This morning I took out a comma and this afternoon I put it back again."