I also got my bike tuned-up recently but I think we had such a long discussion over whether we should change the chain and rear cogs or not that he forgot to tune up the rear derailleur, which was badly in need or realignment. I know he did my brakes though because they carry his signature adjustment (he does not adjust the front and rear brake the exact same way).

I discovered the rear derailleur misalignment on my first ride now that I have left home for the summer.

That mechanics is a family friend and a great guy whom I have a hard time paying for his services. He also charges near-cost for new equipment. I won't give him a hard time for this, I'll just try to get to a bike shop in this area and get it realigned. I like doing my own brakes, but for some reason I am less patient with the derailleur (also is a pain to do without a stand).

So: go back to the store and get it fixed!!