I think I asked a similar question in another thread quite a while back. I decided after reading the replies that it IS possible to overthank.
In my scenario, I went out for dinner with a group of friends (5-6 people), and one fellow (John) had brought his uncle who was visiting. I assumed we were all paying for ourselves, so I didn't hold back on ordering. I ordered a second or third alcoholic drink, I had dessert, and so on. At the end, this uncle of John's, who I'd never met before, picked up the bill and paid the whole shot, for all of us. I thought that was incredibly generous, as for 5-6 people this wasn't cheap, and I was also a little embarrassed because if I'd known someone else was paying I'd have stuck with one drink instead of being extravagant. So I tried to contribute, and when he refused, I thanked him very much, and then later in the evening when we were at John's place, I commented again how generous it had been of him and what a lovely meal, then at the end of the evening as I was leaving I thanked again. John kind of hissed at me, "Look, stop thanking, it's no big deal, OK?" which I thought was kind of rude. But it did make me wonder if I overthanked. And then I ran it by e-hell and I forgot the responses exactly, but I did conclude to stop it with the overthanking (although I do still think a genuine thank you is warranted of course).