I was at a Thai restaurant recently and requested clearly (and the server acknowledged my request) that my curry be made very spicy. It arrived "warm," and I asked the server if there was anything I could add to it at the table to make it hotter, since many Thai places have those little trays with various hot pepper garnishes. She whisked it away instead, and it came back not much changed.
I didn't try again, in part because I was dining with friends, but mostly because I've heard so many warnings against sending food back, especially more than once. The service was otherwise good, and very pleasant. My meal was by no means ruined, but I don't think I'd have send the dish back again even if it had been, because I'd be too nervous to enjoy even an improved dish.
Is it ever safe to send food back? Is tampering with returned food common in kitchens? Does anything (the server's demeanor, for example, or the "fanciness" of the place) give a reliable indication?
Responses from anyone in the business would be especially welcome.