Layering Mascara: Good or Bad?

I have been layering my mascara for years and nothing bad has ever happened to my eyelashes because of it. However, even though some of my friends think its a genius idea, others think it's an absolute no-no because layering different kinds of "tar" on your eyelashes is unhealthy and will cause breakage.  I've never experienced this, abnormally, but I want to know your opinion.

Have you ever tried putting on different kinds of mascara to get a very full effect?

Have you had any bad experiences with certain mascara-combos?

Post in the comments below!  I love all kinds of beauty-advice!