OK, here's my gameplay bugs I think I've found after getting _maybe_ halfway through the game.  

There could be spoilers here, so don't read if you haven't gotten past the first one or two missions.

Feel free to post if you don't think any of these are bugs, or if I am mistaken.  I'm playing on the Consultant skill level.

Here's some bugs from single-player.

1) Enemies will shoot thru walls and trucks at you, as if the obstacle is not there. No bullets fly thru.

2) When enemies fall from balconies they will go through the ground (on Hong Kong level)

3) Some scripted enemies are misaligned with their equipment - on one of the snow levels I saw one or two enemies not lined up with the fixed machine guns, thus they stood there but didn't fire anything.

4) During cutscene when Mullens is with Sam and the lady in the Shop, Sam is seen sitting to the side of the computer keyboard typing away in the air.

5) Assault rifle shots (Colt, AK47) are underpowered in the single-player game, since a single hit from a regular shotgun is enough to stop an enemy in it's tracks, while AR shots take 3 to 4 rounds (on Consultant skill level)

Sure, a shotgun will kill at close range, but the damage inflicted from both types of weapons does not seem consistent - a single hit from an AR should be enough to disable an opponent at close range, too.

The shotgun seems to go right through armored opponents.

6) Enemies on non-stealth missions can sound alarms even though they are not near an alarm.

7) On Snow level some dogs will get stuck at fence openings.

8) Night-Vision and Thermal vision don't allow you to see as far as the enemies can.

9) Enemy accuracy seems about 90-95% accurate on the Consultant skill level.

10) Stun grenades don't have any affect on enemy opponents.

11) In single player you have to throw two grenades at opponents since they will always grab and throw back the first one. While throwing back the first one they are not affected by the blast of the second grenade.

12) While shooting enemies, during their death animation, if you shoot them again the death animation starts over again.

13) On snow level I was able to shoot an enemy who's arms were sticking through a closed door (the machine=gun shack near the frozen river)

14) On snow level there are some areas where player gets stuck while walking. While stuck, if you look down you can see a visible "line" in the floor texture. I assume the player gets stuck on that. If you jump you clear up the sticking effect.

15) In single-player, while firing the crosshair grows to about 4 or 5 times it's size. In multiplayer the crosshair does not exhibit this behavior. The crosshair also grows in thickness, which covers up what you are shooting at. Obviously the enemy AI does not have to deal with this effect.

16) AI can spot you behind bushes. AI detection seems based on direction and distance, whether a hitscan touches the player, and nothing else.

17) Micro-uzis do a lot of damage to both armor and health in single-player, while do reduced damage in multiplayer.

Another one you can add is a line of sight issue with the sniper rifle.

Meaning that if you can see the bad guy, though the sight and half of him is sticking outside the edge of an object, all shots will miss.  But, the bad guy can still shoot and hit you, I think this will go along with your #1.

BUGS:

-red lines appear where tracers would be and dissappear in about a second. Rare and no effect on gameplay.

-hit detection on troops in guardtowers in RMG. Easy shot with sniper rifle but doesn't register until u change your angle and there's nothing blocking the shot. Only effect on gameplay is that it is annoying.

-Invisible walls in mansion level outside around some of the houses. Rare - just annoying.

-Invisible boxes in RMG in base areas. You'll be running and then will act like you stepped on something and nothing is there. My guess is that it is some sort of waypoint for the ai but i don't really know. No effect on gameplay - just distracting.

-In RMG if u snipe a soldier on the ground who is close to a guard tower and within view of the M60 gunner in the guardtower, the M60 gunner will sometimes start shooting his buddy's corpse. He won't stop until u kill him. No effect on gameplay - but really funny to watch.

-In RMG, sometimes if an enemy is shot far away - he'll throw a grenade even though he is about 4 or 5 times beyond his throwing range. No effect on gameplay.

-same as above but about 1/3 the time when he throws it, he doesn't even aim for the player but he seems to throw it at his own guys. Also no effect on gameplay.

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to first post - if u prime the grenade long enough they won't have time to throw it back.

One small note about one of the items:
Hold a grenade for two or three seconds before you throw it, that way the enemy won't have time to toss it back.

At the end of Colombia where you are flying down the river, if you die the black screen to restart or quit doesn't display any text and the music doesn't play. But you can left click and it restarts. And while clearing buildings you can see shadows pop through the wall when someone gets ready to open a door. I've seen a few graphic errors- control panels with the lettering backwards and a computer with the left and right halves interchanged. The biggest problem I've had so far was Hong Kong where you have to jump from one balcony to another. Both times took me a very large number of attempts because the distance is to great. I had always run and always jump/crouch turned on for this part.

Mysterious deaths in the Columbian levels
I swear it's my so called "team" throwing grenades at me and shooting me when clearing the buildings 'cause it sure as #### isn't any of the bad guys

I've seen them toss nades through a wall (seen it at the start of Columbian village).

I've also seen them shooting their own after you killed one of them, like mentioned above.

Once or twice, I've seen dead copses fall into the ground, while being shot while in mid-jump, kinda like above.

I've picked up a body once, and droped it off in a room, and watched it hover. (kinda like dropped weapons hover sometimes in MP)

Other than that, its been kinda light on the oddities I've observed.