Because of the renewed interest in this thread I added some clarification to the title.

This happened to me today but not the way others are describing. I did not take out the take-down lever first.
I had the FCU out just to get an idea of how it worked and somehow the trigger was pulled before I was putting the FCU module in the gun.
After getting the FCU back in the hand grip module, I installed the slide and couldn't get the take-down lever in. I could not get the slide back off either. I was able to pull the slide/FCU back out together. I now realize what I did wrong. I installed the slide before re-installing the take-down lever. I fought with it for an hour then I searched and found this thread. The video that Sleepla8ter posted was a life saver. It only took a few minutes to get them apart.
You don't have to remove the take-down lever first to screw everything up. I will know next time if it ever happens again.
One thing that really surprises me, I gave my Son a brand new Sig P320 and he took it back home with him to Texas. I purchased a new Gas Pedal lever and had it sent to him. He slid the take-down lever out and the new one back in without removing the slide. I am SO surprised.

THIS TOPIC SHOULD BE A STICKY.

^^^Excellent usage of the Search Function by the new guy...

Welcome to SIGforum, 'hurst01'