Idle problem after exhaust mani install

I just replaced my exhaust manifold, and now at the first start it will not hold idle, and will stall out. I can keep revs high for a bit, and it will eventually keep idle.

What did I screw up? I had fuel line and all the connectors off the intake to do the mani swap.

I checked the wires to all the sensors, and I believe the are all ok. This is a 99, so would bad/disconnected sensors trip the check engine?

I hit the O2 sensor wire pretty hard, could that be the problem? (looked nearly cut, but just dented the wire with the downpipe collar when impact wrenching the second bolt off)

Also, no exhaust pipe past the cat shouldn't be a problem since it was running fine without a down pipe.

Oh, I had to replace most of the vacuum lines as well which got melted, but it ran fine without those as well

Thanks for any help

Thankfully noone answered because the problem went away on its own, maybe it was that gremlin i see out of the corner of my eye in my garage every once in a while.