CLose call V belt ruined

Thanks for the comments. Well the belt that flipped was only 2 months old and was basically new. When I removed the Alt, I kept the belt on so it went back on same way. Though I did get it pretty tight. maybe too tight? So I now have the old one on, from before when I replaced it initially. For some reason I kept it in the trunk. When I look at engine as it is running and rev it up by hand, there is a point at which the belt looks like is it deflecting 1/2in in either direction, but as I rev past that point it smoothes out.
Also at low rpm, like when stopped at a light, I hear a rattle like a belt rattle? Or maybe the hood, unable to trace it cause it only happens when I am in gear, stopped.

is it common for the belt to deflect so much? Alignment looks good. the alt pulley is slightly tweaked so it is not spinning 100% straight. very small warp as it spins. Now I need to work up the confidence to get back on the freeway and let it run for a bit and pray the belt doesnt flip or worse, break.