I, too, had the same problem of not being able to remove my slide on my S9. I also had the same issue as shown in the above photo from @Printrep. In fact, that looked just like mine.
Instead of sending it to S&W and waiting a couple of weeks (or more), I went to my local gunsmith and I picked it up in a couple of days. In fact, it only took them a few minutes to fix it, according to them.
The problem is with the tear-down lever retaining wire/spring. That silver piece of metal you see in the photo is the wire. They thought it was broken/bent and they would have to replace it, but they simply put it back into its slot.
I think what caused it was when I was putting the slide back on and did it in a crooked fashion, then trying to remove the slide with a little force to retry inserting it straight. When it finally went back on, everything was fine for a week or so, until I went to clean it. That is when I discovered it was popped out. I guess it's unnoticeable until the next time you attempt to tear down the gun.