Interesting thread particularly as my parents Mk1 leaked like a sieve and i spent months trying to stop it from leaking (well documented on the Mk1 forum). Imagine my horror then when one day I came back to my car (Mk2) and water was literally running down the inside of the screen. Here we go again I thought. However the odd thing is it only happened the once, never before and never since.
That lead me to belive that it must have been a specific set of circumstances that led up to that amount of condensation forming on the inside of the window, although I don't have any idea what it could have been. Touch wood I'm one of the lucky ones and it won't happen again.
All I can say is that I never normally turn the car off in the demist position, always set it back to auto or feet before turning off and never turn it onto demist until it has been running for a minute. Maybe I didn't do that the day it condensed up? Just a thought.