Version 0.9.9, Windows XP, Windows version of DoomRL.
How I did it: Not entirely sure. Two floors ago I ran the wall. I just now noticed my shotgun ammo had two incomplete stacks.
Other effects of bug: Erm, well, none I suppose. I dropped the x39 stack, and when I picked it back up, it filled the x65 stack as typical.
Miscellaneous thoughts: I've been using the Assault Shotgun a lot. I used it a lot during the Wall (but also a lot of rockets). I am certain between my time in the Wall and the picture you see there, I had picked up shotgun ammo, many stacks from both prespawns and from sergeants.
Details that probably don't matter:
*It's an Angel of Impatience run (in case it wasn't obvious).
*I am going Army of the Dead route. I have Reloader, Badass, and all things in-between.
*I had all sorts of windows open while playing.
*I found the Assault Shotty on level 2, what luck!