Is mixing coolants REALLY that bad?

[Riknos]

I replaced a thermostat and temp sensor on a 2004 1.9 TDI Passat on the weekend, lost some coolant so bought some G13 from VW and topped it up. When disposing of the old coolant I noticed it was green, and the VW G12/13 is pink. Googling this, lots of websites said do not mix them under any circumstances as the coolant can turn to a jelly like substance? Anyone ever had this happen? How likely is this?

The car is on 145k and 140k of that was with the previous owner, what is the likelihood of this gelling happening, and if so how quick does it do it? is it weeks or years? Should I rush out tonight to get some more coolant or wait until the weekend to do a full flush? Or not bother and leave it, as the car hasn't got many years left in it to be honest.

Thanks

[Riknos]

Cool cheers guys, I'll order some more G13 in and get it changed on the weekend, car will do 50 miles total before changing it so hopefully should be fine.