python-mode unexpectedly switches to tabs for indenting

[matt <matthew.a.hess@gmail.com>]

For reasons I don't understand, emacs has unexpectedly switched to using tabs instead of spaces for auto-indenting and commands like (newline-and-indent) [key sequence: C-j] for one of of the python files in the project I'm working on.  It was using the 4-space indent that the python mode has configured previously.  I checked and none of the python-mode configuration has changed.  It doesn't happen for any of the other files in the same directory-- just for this one file-- which makes me thing that I somehow unwittingly set some file-local variables.  The file has DOS style newlines.

I tried using the "M-x untabify" command and then query-replacing all the tabs in the file to see if that might toggle the behavior, but tab insertion on C-j still persists.

I'm really at a loss to explain what happened, and it really disturbs me that this can happen without me having any clue.  Any help/insight is appreciated!  Thanks in advance!