Pic shows much more ammo than i actually should have with that weapon =>
I have field ops #1 weapon equipped here, Basically what i did was throw an ammo pack right in front of me (this btw causes another bug i describe in the thread i will soon open), then shoot my weapon once, then let my ammo pack pick up while i reload, then my count goes randomly up a certain number.
On the pic i have achieved +34 more ammo. Who can do more gets a cookie 
Here is another thing fitting into that category: throw an ammo pack widely in front of you, so that you are out of its bounding box, just normal throwing. Then step over it and empty one full magazine (i tested only fieldops weapn #1 here). You will notice that you will pick up the ammo pack, but absolutely no ammo is added to your inventory.
EDIT i correct myself, the ammo is added logically, but it is not visible in the HUD. After i reload it is visible again. Still, this is a bug.
Another thing: i noticed this but did not test entirely to be 100% sure. Tested this with medic primary gun. Empty all ammo, then wait for a fieldops to throw 2 packs. Choose another weapon so that it does not autoreload. Then walk over the two packs.
What should happen is that you will only be able to pack up med pack number 1. The second can only be picked up when you reload your primary.
It happend once that I reloaded, had 14 bullets in my magazin and 15 bullets for reload and after reload i had 30 bullets in my magazine. Its new to me that 14+15 = 30 
Indeed happened to me several times, also on a lot of ohter small examples i do not remember that much, but i remember there was something weird with the counting several of times.
I thought 35 + 70 Bullets is the max, picked up from the ammo rack when I had 0 bullets left:
[0265.00] Log: Log file open, 03/19/13 20:41:38
[0265.00] Log: Dumping BugIt data chart at 2013.03.19-20.41.38 using build 8710 built from changelist 17437
[0265.00] Log: MapName: OBJ_Victoria
[0265.00] Log: Description:
[0265.00] Log: BugItGo 4920.8887 -1173.5911 2156.5305 -3778 -17055 0
[0265.00] Log: ?BugLoc=4920.89,-1173.59,2156.53?BugRot=-3778,-17055,0
[0265.00] Log: ---=== GameSpecificData ===---
[0265.00] Log: Log file closed, 03/19/13 20:41:38
When I switched to pistol and back to the gun, the ammo count was normal (35 + 70)