This has happened to me a few times and it has to be one of those "wtf" moments
Sometimes I have found cancel thelf suddenly having 2-3 mortars that I'm sure I didn't build. It must have happened while drag-selecting your units in the mainbase and trying to reinforce using the R hotkey.
What happens instead is you end up selecting the T1/T2 structure and you end up with a couple of mortars without noticing the screw up during an intense game
What's yours?
Thats funny I have had that same thing happen to me quite a few times, even notice once i was building two motars but was unable to cancel them. I think it mite be a bug