By default, Idle runs user code in a separate process from the idle process that runs shell and edit windows. Because connecting the two processes sometimes fails, there is a start-up option -N (No subprocess) that run your code in the same process as idle code. Since this option also has potential problems, it should not be used unless necessary. In fact, some of us would like to eliminate the option (after fixing connection problems).

I tried and failed to reproduce the problem with 2.7.5 on Windows. I started with -N, opened an editor window, ran (F5) with a comment uncommented, got a syntax error box, added '#', ran again, and the valid code ran fine.

To continue this issue, update from 2.7.3 to 2.7.5 and see if you still have the problem. The update has 100s of bug fixes including many Idle fixes. If you do still see a problem with 2.7.5, please find a minimal file that has creates a problem when '#' is missing. Also, if you do, please test with 3.3.2 or the most recent 3.4.0.