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.
There are a couple things that can be going on here.
It's possible to adjust a front derailer, or more specifically some derailer/crank/chainring/chain/shifter combinations, in a way where it can seem fine most of the time and doesn't have any obvious problems with the derailer position, but is prone to jamming the chain, which can in turn cause the chain to get damaged or break. It's a problem that for the most part has to be uncovered by test-riding post-adjustment and making further adjustments, usually to the derailer positioning, as needed on a case-by-case basis.
Another thing that can cause what you experienced is the chain breaking due to prior damage, which can include being rejoined in a manner that weakened it. When chain manufacturers say to only use their special connection links/pins and not re-press pins on modern chains the traditional way, what they're trying to prevent is the exact kind of breakage you experienced. I had my only chain breakage ever when early on in my cycling life I re-pressed a pin on my SRAM PC48 instead of using the Powerlink for some bad reason, and it broke very shortly thereafter during a front shift.
Finally, there is a limit to how much load a chain can endure while shifting, both as a habit and during single incidences. Pushing that limit can result in a break even without any other issues and regardless of what any mechanic did.