A couple of times I've experienced an insane temperature readout on my AMD Phenom II X4 955 BE CPU saying 256C.

Normally Low is 36C-38C and high 43C, but when this happens High value of course changes to exactly this: "256(!)". When it happens it is also written in red text momentarily.

I don't quite understand how this can happen

This should be fixed in the final release.