I built T1 with Soviets and pressed the "S" button for a Sniper squad as soon as it finished building. It finished and the sniper was being built but suddenly the building was destroyed. So I lost my T1 aswell as the manpower for the sniper investment.
I was like
I guess it had something to do with the hotkey for cancelling the contruction of the building still beeing active (it was "S" aswell).