Is it me or is there a bug?

When I use the general format for cells, I find that after the file is saved
and reopened these cells go to a custom format
[$-C09]dd-mmm;@
These cells are in columns, with one column of single or simple digit
numbers, and another column of long text. The long text consequently displays
as XXXXXXX, rather than a much more friendly overflow over the adjacent
cells.
And the single digit numbers then display as a date, which really is
frustrating.

I understand that Excel will try to find a suitable format if it is set to
general, but this is something different.

I suspected this problem first with a file that had it's name changed in
explorer, now I am finding it in a few others that simply opened and closed
normally.

Your thoughts? Willy