[V8] major issue today with new v8

[chris brown]

Today i went to get my car at my mechanic. I different guy who i trust a lot more than who did the initial swap. He found that one of my cylinders had no fuel getting to it due to a injector connection that was not hooked up properly. They cylinder had good compression like 200 lbs. when he hooked up the connection for the injector the motor began to fire on all 8 and sounded smooth and seemed like i was in business. As far as i know he didnt do anything else. I left the shop with the car and i didnt get about a half mile when the car lost all power and it began to run horrible. I couldnt get it up to even 40 miles per hour. The CAT light came on and the CAT itself got red hot and began dripping a liquid that appears to be oil. It was actually so hot that as the liquid dripped from the CAT it was on fire. Little balls of fire dripping. It smoked really bad for a while.  I just found it odd that the CAT went bad almost at the exact same time he found the cylinder problem, although i know its possible. 
 What would cause oil to get into my cat and for it to get so hot that is smoked and caught fire?   I was very lucky not to lose the car if i had not turned around and took it back to the shop immediately it would have caught fire for sure.   I really need advice on this one. 
  Ive been driving the car for about 2 weeks now occasionaly and i never had a CAT issue at all. It actually ran pretty good on the 7 good cylinders.  Basically we hooked up cylinder 8 and it smoothed out the motor, then almost immediately the cat clogged  and almost caught fire. 
 thanks chris