My guess would be you are opening something created in 2.71 or prior to in 2.73. Same problem on Windows. If that’s the case don’t do it. Finish those files in the version created in or on Windows no higher then 2.70 I think it is but could be wrong. I believe I remember taking a file created in 2.69 into 2.72 and screwed it up. I’m not a techno by any means but had to chime in.
always play it safe. make a copy of the file and only work with the copy, never work the original. that way if it breaks rather than pulling your hair out trying to fix it you can just make a new copy to work on. try useing 2.71 and get as far as you can with that so the work the script does is finished then if your project is still unfinished because of something you need in 2.73 save a blend and name it XXXXXtrailcopyfor273 or something then open that new file with 2.73 and see if you can complete it without breaking the file.