Unexpected strong fog during a flight

Hi,
After took off I get into extreme fog it was two times. Before flight of course the weather was checked (VFR) condition wasn't perfect (vis ok, clouds) but ok for VFR. Then during flight I a very strong fog suddenly appeared (from 3500 ft to the ground). First time I thought that it was an error of checking the weather, but second time I didn't believe and took off with F-18 and flight over that place - NO FOG!

Both flights were in vatsim and two times I crashed (VFR and airports for VFR only) :evil:
I have no idea how to explain it, didn't see it before. But I consider to use this program until I will sure for solving a problem.

Any ideas?