Filesystem is read-write but acts like read-only... remounting fixed it?

Most likely you had a disk IO error. Check /var/log/messages. Look for messages about aborting the journal and re-mounting read-only.

P.S.: If that is the case you best have backups of your data.

The problem with your diagnostic steps: if your root filesystem is read-only, /etc/mtab is also read-only. Guess what mount reads to get filesystem status :)

The solution is to check /proc/mounts, which is a dynamic kernel file that shows the true filesystem state. If you'd have done so at the time you had this problem, you'd very likely have found your root filesystem was remounted read-only. Know your utilities and where they get their data from.

How's that happen? Most /etc/fstab files will have a line similar to:

/dev/sda1 /               ext3    relatime,errors=remount-ro 0

Note the "errors=remount-ro" option. In the event of any filesystem errors, the root filesystem is remounted in read-only mode.

Assuming /var/logs was writeable, or you had offline syslogging enabled, you should find the event logged as well.

Disk errors capable of triggering a remount-ro are generally a Bad Thing. Hope you swapped your drive.