By the way, a sometimes I've done something similar with documents I write for work. When sending them out for review, I put in a line of text saying, "If you're the first reviewer to mention this sentence, I'll buy you lunch." I'd say about half the time, I get called on it. I can't remember where i got the idea - I think it was at my first 'real' job at a transit/traffic consulting firm where someone did this.
Hope you made out better than I did doing almost the same thing at my first real job.
I put a single line in the middle of a paragraph about halfway into a twenty page financial analysis I had just completed. I had offered to buy and bring the first person who saw it their morning coffee for an entire month.
I got called into my manager's office about an hour after I sent it up for approval. After being reminded we were expected to "act professionally at all times" (since this was the "grownup world") he strongly cautioned me against pulling any further "college pranks" if I planned on continuing to work there. I was then sent back to my desk with a red face and my tail between my legs.
But not before my manager also reminded me that, per my offer, I now owed him a month's worth of free coffee. (Black, two sugars please?)
(He made sure he collected on it too.)