I got a very unexpected upgrade at the gate on an LHR-FCO flight this past Saturday. I was flying on an Avios booking in Y but was one of several whose boarding cards wouldn't scan at the gate. All of us had been upgraded. There were delays, etc, that morning; so I'd guess the overall busy-ness caused them to include me in the bump ups.
It was my first ever upgrade in Short Haul. TBH I was a little annoyed to lose my exit row seat, but the champagne and very nice meal (CE meal service has improved a lot lately) helped take my mind off the slight discomfort.