Error opening txt file on MacOSx
Without seeing the screencap, it really sounds like a filesystem error to me, yeah I could be full of little red ants but the first thing I would do is open disk utility and do two things, repair permissions and verify the disk (you cant repair it, repair will be greyed out).

If permissions repair works you are golden, if the disk does not pass verification, boot into recovery mode (reboot holding COMMAND-R) and run disk utility and repair the disk.

If that doesn't fix it (EG Disk Utility returns an error message) it may be time for a third party repair tool like Diskwarrior or Drive Genius. If THAT fails then it's time to backup, reformat and reinstall the O/S and restore your Apps and files... You do have a backup, don't you?