Bugs in Microsoft Office, and an answer in Ask Different
/
The / bug is one of a number of old bugs; Microsoft has not properly updated apps such as Excel to work with POSIX-compliant use of the HFS Plus file system by Mac OS X. It's vaguely possible that no-one has reported the bugs to Microsoft.
Other explanations
In Ask Different: Excel 2011 Opens Spreadsheet As Read-Only today's answer from me goes beyond the / example.
The opening poster's case
If the symptom recurs, at the previously affected path, try ls and lsof as suggested in Ask Different.
/
The / bug is one of a number of old bugs; Microsoft has not properly updated apps such as Excel to work with POSIX-compliant use of the HFS Plus file system by Mac OS X. It's vaguely possible that no-one has reported the bugs to Microsoft.
Other explanations
In Ask Different: Excel 2011 Opens Spreadsheet As Read-Only today's answer from me goes beyond the / example.
The opening poster's case
save it to a different place, did it straight to the desktop and it's fine :/
If the symptom recurs, at the previously affected path, try ls and lsof as suggested in Ask Different.