Sephora is all about samples, and every time I talk to an associate there, it's always "can I make you some samples?"
They have a fantastic, no-questions-asked return policy. But no doubt it's more cost effective to load you down with samples than to return the five foundations you've bought and don't like because they oxidize, or don't match in natural light, etc. You don't even need a medical excuse to ask for samples from them. Most of the time, they pull the samples from the testers already on the store floor, and they've already set those aside for testing. You taking a sample away is the same as you putting on foundation in the store and then wiping it off immediately.
If you're feeling like a gimme pig, make sure you remember the associate's name. Write it down, maybe. And then if you buy something, even at a later date, there's a survey on your receipt that you can take to mention the associate and what they did by name.