I personally ran into this on a Sun workstation. The earlier versions of the "format" command didn't do the partitioning math for you. So you had to add cylinde

I personally ran into this on a Sun workstation. The earlier versions of the "format" command didn't do the partitioning math for you. So you had to add cylinder numbers yourself. In calculating the start position of /usr, which came immediately after swap, someone had forgotten to carry a 1. When displaying the partition table, if I remember right it only showed starting position and size. So to verify it was correct when looking at it, you had to do the math again, so it was difficult to spot the error. But it had the exact symptoms you describe. The user complained that the system worked fine until they opened a lot of stuff, then it crashed. The only reason I figured it out was that we had several identical machines (with identical disk drives), and I happened to run "df", and this machine had a suspiciously large amount of disk space. Space was always tight because disks were small (200-400MB), but none of the filesystems seemed close to filling up. I couldn't explain that, so I started scrutinizing very carefully, and then it hit me.