Which behavior do you propose to change? Does PEP263 specify the response to a self-contradictory encoding comment? What do you think it should say?

I raising is the better behavior. 

Idle notices that it cannot save the file with iso8859-15 encoding, so it saves it with utf-8 encoding instead. Any Python-aware editor that recognizes and uses the encoding cookie would have to either do the same or refuse to save until the encoding specification were changed.

Trying to run the file brings up a error box with SyntaxError: encoding problem: utf-8. Note that Idle runs a syntax check before creating a new subprocess, connecting to it, and trying to actually run the file. Closing the box takes one back to the editor as the spot where the error was noticed, which in this case is at the end of the line.