Sehr interessante Frage inklusive ausführlicher Antwort auf Reddit: What would happen if you stuck a lightsaber into a mirror? Spoiler: Kommt drauf an.
If you're in a non-oxygen/nitrogen atmosphere (or the lightsaber is not tuned to the atmosphere you are currently in):
The plasma-generation laser passes through the air imparting minimal energy (instead of all/most of its energy) to it, strikes the reflective surface and, depending on the angle the lightsaber is held at, either
A) passes back into the lightsaber, likely leading to a catastrophic explosion and/or grievous harm to the user
B) reflects off of the mirror and continues on to cause catastrophic damage to whatever it finally strikes