I had that lockup problem (unloaded in my case) a couple of times with my M&P 9 when it was new. I had to force the takedown lever downward in to the verticle position to get it to so I could move the slide.

After it happened to me a second time I decided to look for a cause. I noticed a burr on the edge of the half round edge of the takedown lever shaft. I used a gunsmith file to knock it down. Then I used a fine stone to break the edges of the half round section of the take down lever. This seemed make it less likely the the slide would hang up on the lever. I also did a little polishing on the edges and flat surface adjacent to the half-round part of the lever. More importantly, I now make it a point to hold the lever firmly in the verticle position when installing or removing the slide.

When I recently bought a new M&P9c, I dressed up the edges of the takedown lever the first time I took the gun apart to clean it before my initial range trip.

I have to tell you that I didn't have to deal with this issue with a round loaded into the chamber. That is too scary to think about. Like some others above, I never load a gun until I have function tested it, and I never load anything while it is being worked on or cleaned. I also use dummy rounds to function test things (either snap caps which you need to buy, or dummies that I make up and keep on my reloading bench).

Anyway, I am sure glad that you were able clear this up without a tragic mishap.

By the way, I have often wondered why the guys at Smith didn't do a better job of finishing the take down levers.

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Andy