Usually what they do is buy the new bulb, take it home switch the bulbs and then return the old burnt out one for a refund in the carton the new one was in.My thought exactly.
So let me play this out so I can understand.
My T-8 bulb burned out and I'm too cheap to buy a new one. So I walk into the pet store carrying the bad bulb, go to the lighting section and, in plain sight of god and everyone, I open a new bulb box, remove the new bulb and insert my bad bulb into the box. Wouldn't it be just as easy to walk into the pet store and remove a new bulb and walk out with it? Either way you're carrying a bulb. I don't get it. They must do it for the rush.
And as far as the finding a burned out bulb in the new box goes, if you have your receipt all you need do is take it back to the store.
When they try this at the fish store I work at it doesnt work. We test every bulb before it leaves the store.