Can a gear change snap my chain? On a ride yesterday I changed from my large to small cog on the front gear set and my chain instantaneously snapped. The snap occurred right on the point where the mech would have applied pressure.
The previous day I had a gear service from a mechanic who now swears that this is pure coincidence but I would prefer opinions from people who aren't trying to save money and reputation. 
I was already considering returning my bike for a re-service as the outside of the front derailler was still rubbing the chain when in the smallest back cogs.
Could it be a case of poor adjustment work that placed too much strain on my chain during the gear change (I was only in gear 3 on the back)?
This was also the first time I used the small cog since the service
A chain is in danger of snapping when it develops a weakness. It will fail when the force on it exceeds the strength of the weakest park.

The rider might exceeds the strength of a weakened chain through high-force pedaling stokes, when accelerating from a standstill perhaps. 

Another way the strength might be exceeded is through a short but high 'shock' load on the chain, that might be induced by a gear change, but without very high pedaling force. Think of the 'clunks' you sometimes get when changing to a smaller sprocket in the rear.

What I suspect happened in your case is that the front derailleur imparted a lateral bending force on the chain at a weak point which in conjunction with pedaling force caused it to fail.