You can have problems with even the best run and most honest shops.
After I acquired my '90 Westy a year ago and just before a long trip, I
took my van to a very trusted (and good personal friend) for the basics
(tune up, flush and re-fill all fluids, etc) and had his shop put on a
new set of shocks. The tune-up included new plugs, cap, rotor, air
filter and general very careful goingover.  They did find a leak in one
component of the cooling system and replaced it, topped off and bled
with approved coolant.  Everything else checkd out fine.  Just for
kicks, I picked up a K&N air filter and a set of the multi-pronged Bosch
plugs, took them along on the trip, and planned on installing them about
half way through the trip to see if there was any improvement in gas
mileage one the return trip.  When I went to install the plugs, I found
one was only finger tight, had not been tightened by the tech.  In his
checkout of the engine compartment, he had missed the fact that the
crankcase breather hose was rotted through and crumbling, even though it
is right on top of the engine.  In addition, the air filter housing was
not installed properly, was rattling and not sealed.
I corrected all these problems (with $1.00 worth of hose from the local
FLAPS for the crankcase vent) and went on my merry way.  BTW, mileage
did not change with the K&N or 'special' plugs.  They are still in and
serving well, but no miracles.  When I got home I stopped by my friends
shop and politely told him about the slightly sloppy work.  I had
noticed that the tech that did the work was no loner there even though
he had been with Karl for several years.  He had been a very reliable
employee for a long time, but for unknown reasons had grown sloppy over
a period of about two months (other problems more serious than mine) and
he had been fired.
I only tell this rather long tale to point out that even the best shops
can have occasional problems.  It's how they handle them that counts.