http://www.youtube.com/watch?v=8d_CHC5fXc0
It's real, and needs to be fixed.
OMG! exactly the same ... is so annoying!!!! Arrrrgggggg!!!!!
Is there any way to send it directly to the developers?.
In other Spanish forum there are many more people with the same problem ... is real, is the game.
Please Fix it!!!!!
I found a way to temporary fix the problem:
When you see it, just press Start, quit to the Main Menu, and reload the Campaign Chapter you were playing from the beginning with "Repeat Mission". You will get the default weapons of the level, but the bug will disappear.
It seems to be a problem with "Resumed Savegames" in the middle of the levels...
Let me know if it works for you. It worked for me

P.S.
If you see the problem in Multiplayer too, just do the same loading any chapter of the game from the beginning, let the game auto-save/checkpoint, and then try again going in Multiplayer.