2002 Chevy Venture manifold gasket damage replacing thermos

I took my vehicle to a shop for a thermostat replacement. There were no coolant leaks just low heat and no heat on idle and temp. gauge barely moved. Got vehicle back and everything seemed fine until upon pulling into my driveway I smelled burning coolant. I took the vehicle back and was told that I had a leaking manifold gasket and $1200 repair! No leak upon arrival at shop and no report of it after repair from the shop but major one upon leaving and I was told that it was impossible to have done that damage during this repair, what do you think? Is it possible?