[linux] ubuntu "No space left on device" but there is tons of space

I am getting the "No space left on device" error for pretty much anything i try to do. Even using tab to autocomplete a command!

but when i do df -h i get:

ubuntu@ip-10-0-2-108:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/xvda1       99G  6.5G   88G   7% /
udev            3.7G  8.0K  3.7G   1% /dev
tmpfs           1.5G  184K  1.5G   1% /run
none            5.0M     0  5.0M   0% /run/lock
none            3.7G     0  3.7G   0% /run/shm
none            100M     0  100M   0% /run/user
/dev/xvdb       414G  199M  393G   1% /mnt
overflow        1.0M  1.0M     0 100% /tmp

which to me looks like there is tons of space. df -i also looks similar:

ubuntu@ip-10-0-2-108:~$ df -i
Filesystem       Inodes IUsed    IFree IUse% Mounted on
/dev/xvda1      6553600 94227  6459373    2% /
udev             951353   393   950960    1% /dev
tmpfs            953649   274   953375    1% /run
none             953649     3   953646    1% /run/lock
none             953649     1   953648    1% /run/shm
none             953649     1   953648    1% /run/user
/dev/xvdb      27525120    11 27525109    1% /mnt
overflow         953649    12   953637    1% /tmp

I am on an Amazon EC2 ubuntu 12.04 instance.

Here are some examples of the error popping up:

ubuntu@ip-10-0-2-108:~$ sudo crontab -e
/tmp/crontab.RvYjrR/crontab: No space left on device

ubuntu@ip-10-0-2-108:~$ ls /va (hit tab for autocomplete)

-bash: cannot create temp file for here-document: No space left on device
-bash: cannot create temp file for here-document: No space left on device

however the server seems to be running and everything seems to be working. what on earth is going on??