I've had it happen a few times while flying on CO. I have spent a lot of time on other airlines waiting at the gate for my name to be called for an upgrade; when it didn't come through, boarded and got to check luggage I normally would have had to put in the overhead compartment.

Not related, but I have had pretty good luck getting confirmed upgrades on CO prior to boarding, both as a gold and as a platinum. I used to wait in the boarding area to see if the upgrade was going to clear at least 1/3 of the time; now, almost never.