Bad tune-up...
 

So I took my car in this morning to get it tuned up, in order to help
it pass the smog test. At least the oil change, I thought, might help.

It was running fine when I took it in to the garage today, though, and
when I picked it up afterwards (plugs oil filter and air filter and all
that) (Hyundai Accent GT fuel injected engine) it seemed to be running
maybe a bit better, not much difference, but it was running fine,
before.

So I drove it about 5 miles to work and parked it, then 3 hours later I
drove it to lunch. As soon as I started it to go to lunch, the CHECK
ENGINE light came on AND it was running rough, almost dying at stop
lights.

I am going to take it back to the mechanic tomorrow, but I want to
know: What could have happened? Why did it only start running bad AFTER
I left the garage and let it sit for 3 hours? What might be wrong with
it that he would have done to cause the check engine light to come on
and run rough, only AFTER sitting in the garage at work for 3 hours?

What is the mechanic likely to say? Can he in any way claim it was NOT
a result of something he did wrong?

What can you tell me to help me resolve this without my paying more
money, getting taken for more parts, etc.? I'm looking for knowledge,
here; knowledge is power.