The risks of getting greedy for miles

I just dodged a nasty little trap over at the United Airlines website, and thought it was worth passing on.
About a week ago I received an email from United advertising their “Fall into bonus miles” program. It works like this: register on their website with the special code (MPD539), then enter that code when purchasing a qualifying roundtrip this fall, and get an extra 2,500 miles. It sounded like a sweet deal, so I registered.
It also happened that I need to visit the UK in December. It’s partly family stuff and partly business, and I have a little flexibility about dates, so I plugged in the dates, checked “Search by flexible dates”, and… Ah, yes! Before I clicked “Find”, I carefully entered the code “MPD539”. Back came a matrix of fares for various date combinations, and the best (December 7-21) was around $900 per person.
I wasn’t quite ready to book the trip, so a couple of days later I looked again. This time I forgot to enter the promotion code, and I was delighted to see that the prices had gone down, to around $700. Great! Isn’t yield management a wonderful thing? I still had one thing to confirm, so I waited until I got home to actually book the flights. I logged in, entered the dates, added the promotion code… and the price was up over $900.
On a whim, I tried my search again, without the promotion code. Bingo! I got the $700 (plus taxes) fare. So I booked it, and then checked to figure out what had happened. It turns out that the fine print on the promotion says:

Qualifying class of service: United First(R) (F, A, P), United Business(R) (C, D, Z) and select United Economy(R) (Y, B, M, H, E, U, Q). Other classes of service are not eligible for this offer.

So each time I entered the promotional code, United only showed me fares from the eligible classes of service, and gave no indication that cheaper fares were available. My nice cheap fare was Economy “S”, which may be the bottom of the barrel, but I still got nice seats in Economy Plus.
The moral of the tale: if someone offers you a cool promotion, check the prices with and without the promo code. You may be surprised.