Several years ago when laser diodes first became available to the general public, I purchased a 5mw 650nm red LD for an exhorbitant price. I also purchased a collimator assembly separately from another vendor.
I mounted the LD in the collimator and powered it up - nice bright but defocused red beam. So I decided to adjust it while it was running, but because I hadn't tightened the diode mount adequately, the diode itself had rotated within the assembly briefly while it was powered up and I was fooling around with the lens.
This introduced multiple power glitches into the diode and ultimately blew it out. It then became a very dim, plain old red LED, weakly putting out regular non-coherent light.
And this was after I only had the diode in my hands for less than 30 minutes! Damn. /ubbthreads/images/graemlins/banghead.gif
The moral of this story: handle LDs gently while they're powered up.