Fortunately this was pre-AdSense days when I was pushing Amazon as my principal source of revenue.
I had this "sophisticated" way of includes etc. etc. for a rotating banner ad which called another .htm file embedded within an IFrame.
I was sooooo super smart, didn't want this .htm file to be indexed by SE's so I put in the <head> of that file.
<meta name="robots" content="noindex,nofollow">
Of course, being an include....
Dumb, dumb, dumb. Took days to discover the error, never thought to look at my source from a page loaded from the internet.
A friend picked it up. Dumbest mistake I ever made.