I've seen this happen so many times at Star Parties, that it no longer surprises me when it does.
Primarily through Schmidt Cassegrains, and refractors but Newtonians, and dobsonians are not excluded.
Heck, it's even happened to me.
Someone will be enjoying the views through their telescope, then all of a sudden the image in the eyepiece
Yep, one moment Saturn is there, the next moment it's gone!
Well, the dewpoint on the telescope was reached, and condensation forms on the objective lens of a refractor,
or the corrector plate of an SCT, or even the secondary mirror of a newtonian/dobsonian.
That's why it is imperative that you have dew protection equipment on your telescope. At the very least a dewshield,
or your telescope is about as useful as Mayor Rob Ford showing up at an AA meeting.
If you are not sure what you need for your particular model, then give us a call at 1 800 580 7160 or 416 783 4140 local
and we will be more than happy to assist with you your needs.
Ray "Dew you need me" Khan
PS In Canada we are prone to dew problems, most of the year round. The only way to avoid it is to move to a very dry climate.