I'm guessing this was fixed in a patch or something, because ...
Overflow cannot exceed HPT. The only way to accumulate overflow greater than your production potential is by chopping or temporarily increasing your production by assigning excessive priests/engineers and miners.
really?
My computer didn't seem to notice this, and I've patched as far as possible.
IMHO you didn't test before stating this, because I've had 81 hammers overflow + forge bonus going to the great library in a size 4 city.
I don't believe my size 4 city had 81 hpt

Here is a very easy thing to test if your statement is true or not :
- pure commerce city (=1 hpt)
- start a library
- after 2 or 3 turns of work, whip it
- check the overflow. if it's only 1 hammer, then you're right and I'll stop posting altogether
