1995 mx6 2.0

Hello Everyone,
 
My name is Israel Delgado and I am a car weekend mechanic as a hobby.  Presently working on a 1962 Bonneville(very nice).
 
I had the crankshaft pulley on a 2.0 1995 Mazda eat away at the retaining key.  I replaced it and now, 3 months later,  the alt belt fell off while driving.  I am hoping that I just forgot to tighten it properly.  However, any other possible reasons for the 2.0 to let go of the alt belts and specially to eat away at the crankshaft pulley key.
 
Thank you all.
 
Regards,
 
Israel