It is interesting that this started out as a thread on silly frugality, but has mostly morphed into a thread of just regular frugal things that people do.
Now, I've mostly stayed out of it as I don't consider myself particularly frugal. To me, though, silly frugality is different from regular frugality. Something is "silly" if the amount of money it saves you is not worth your time (as you judge it) or if the frugality negatively impacts your quality of life.
I sit here with the thermostat in my office set at 78. I like to be warm. I recently visited my very frugal mom and had to beg her to put her thermostat up to 60. I was miserable the entire time I was in her house (yes, I had a jacket on and blankets on my legs). It is not worth it to me to set my thermostat down as it would negatively impact my quality of life. (As others have mentioned, I save on the flip side. I don't like cold AC in the summer).
As far as leftovers. Eating leftovers is fine. I am sure most people do it at times. There are two times when I think the frugality is silly. First, from a calorie standpoint. Sometimes I go out and eat a high calorie meal. I can afford to eat a normal size portion of that meal. But, if I bring home the leftovers then I end up spending more on the meal than I wanted to spend -- that is spend more calories which was just as important for me to watch as money.
The other times eating leftovers are silly is when you don't want to eat them. If I feel that I have to eat the chicken that I have leftover from a previous meal but I didn't actually like the meal then it is silly for me to eat it.