Top reason i have seen is from improper removal of the transmission. Ive seen folks try to remove the transmission without unbolting and pushing back the torque converter... that can put the weight of the tranny on the snout/tabs and crack it or snap em off...

1970 Charger - V10, 6spd, Alterkation, Street Lynx 4 Link, Moser Dana 60, Wilwood 14" disks, Forgeline 18" Wheels (Rear:335's), ISIS Multiplexing Wiring http://www.v10mopar.com 2012 Charger SRT8