After reading the article History of Entropy on Wikipedia, I noticed that it mentioned Carnot running into problems with entropy. I understand that entropy was fleshed out later by Clausius, but with respect to Carnot, what experiment did he perform that gave him reason to believe there was something else (although he didn't actually figure it out). I'm not looking for any answer that includes Boltzmann and probability, or any answer that includes the word "chaos".
TLDR: What did Carnot do or see that made him think there was something missing? What happened, possibly through an experiment or even thought experiment that made him realize things weren't adding up?