In the lobby shootout, Chapter 29, time index 1:42:43, check out the profile of the high-cap magazines in Neo's pistols. They're double stacked 9 mm mags. Five seconds later, (1:42:48), the camera angle is a floor level shot of the spent casings falling as Neo fires the pistols.

The casings are .223 Remington, which are wrong for Neo's guns, but right for the M-16's in use by the soldiers. For that matter, there are way too many of them clustered together in the slow motion shot, and they're falling straight down.

Normally spent cases are ejected to the right

I suspect they had a drop bag on Keanu for the cases, and filled it with the wrong cases.