a) I've only had this computer for 8 months and figured 500GB hard drive would be fine because...
b) All of my documents, music, photos etc are stored in the cloud and...
c) All of my video work is stored solely on external HDDs and in Dropbox.
However, after a bit of minor sleuthing, I tracked down a 13 hour screen recording in my system library folder which was around 150GB
Scratch disk is no longer nearly full
Only question I have is how on earth it ended up there. I do QuickTime screen recordings a lot, and they save to desktop, so the only thing I can imagine is that the computer ran out of battery and it aborted the recording which meant it ended up in there for some reason.