I know of some instances of optics being shot and cracking (red dots). however magnified optics (scopes) are generally made of pretty dense glass so unless they're really bad quality, or getting shot by some really hot gun from close up, they'll most likely be okay.
For example, most airsoft won't break windows (even though they're much larger surfaces and nto as thick as most optics) or glass bottles. The CO2 powered KWC Mini Uzi however does fire extremely hot (too hot to be gamed with without a conversion to lower power gas) and it can break a bottle (as seen in a video review of it), so that would probably damage optics. But it would also injure someone quite a bit (compared to "field legal velocity" guns) so it's not really an issue I guess.