usually in a lucid dream you don't see your reflection, like how your fingers might be too long or something, so your mind has to compensate for that
usually you think of something scary in a mirror and you see it, but it might be your real reflection 

So if I look in a mirror, I'll have elongated fingers/elongated extremeties?
A double-edged sword!
I tried this once. I've never actually done it, but almost went into sleep paralysis.
stuff was scary; it felt like my chest was being compressed or collapsing or something, and I heard my own heartbeat.
Kind of interesting at the same time though.