I had interesting things happen when I ran out of disk space today.
The most notable one was that I saw “Process completed” — or some variant of that — every time I tried to open a Terminal window with a new shell session. I briefly staved it off by specifying Bash as my shell, but then it came right back after opening another tab or two in Terminal. Consulting Google led me to this “Terminal application quits” thread at Apple Discussions. On a lark, I tried deleting /usr/bin/login as one poster suggested. It worked!
…But only for a little while. The problem returned. In the meantime, I had freed up some disk space because I’d realized I couldn’t save files anywhere (“But ~/Pictures is writable!”). Clearly something else had become an issue, because disk space was available.
Then I found another thread, “Terminal’s ’Process Completed’ message and /usr/bin/login,” on Apple Discussions. The more permanent solution from that thread appears to be the removal of corrupted Apple System Log databases. Once I did that and restarted the ASL service, all was well and has stayed that way so far.
Filling up my disk must have corrupted the logs as they were being written or rotated, and led to this cascade failure. Like I said, interesting!
One of the posters in the second Apple Discussions thread indicated that the underlying database corruption issue is addressed in Snow Leopard. However, it seems that you could still see this on Leopard — my experience was with a recently-patched Mac OS X 10.5.8 system.