Well it's actually possible. I don't have that much trust in bike mechanics in general since they usually need to do the stuff quickly rather than do a good job of it. I've had issues with every bike mechanic's work I've ever tried and have since started doing all of my own work and am much happier. Most bike mechanics also mount hydro discs wrong due to the industry pushing the wrong quick and dirty method instead of teaching the correct way of doing it, which takes significantly longer, but afterwards you never have to touch the caliper again.

For instance, have you ever had a mechanic true and properly tension a wheel? I haven't. The wheels were always straight as arrows but the tensions were all over the place.
I've had badly adjusted rim brakes, the most extreme example of which was where one rim brake pad had a toe in angle of about 30 degrees.
I've had badly adjusted bearings
I've had a pedal destroyed by an LBS (and it wasn't a cheap one at that)
Etc etc.
The fact that someone works at a bike shop is not yet any kind of qualifier of decent work. Then add the time component and what you'll likely get is crap.

Or you just had oil on you pads/discs and never noticed.