The installer is hardcoded to download upgrade images to /tmp. Currently, 1.4 and rolling release images are about 515M. On a machine with 1GB RAM and moderate load, there is often just short of 500M in /tmp, so such machines are impossible to upgrade without killing some processes or editing the script to use a location in persistent storage.
I believe we should make the script always download the image to persistent storage. /var/tmp would be a perfect candidate but it's, oddly, also mounted to tmpfs.
There is also a problem with hardcoded download paths that if two people try to upgrade the system at once, those scripts will try to write to the same file. A proper solution should probably also use a unique path, with a random number of PID in the path.