Yeah I had that happen too, on the same mission - although in my case it was just the one gun that did it. It was right at the start of the mission that what you described happened, so I restarted it and it didn't happen again. It might have something to do with the proximity of the gun to the sandbags maybe?
Another animation error I had was a lone American solider who 'lost' his animation completely, and was represented as standing for the rest of the battle, regardless of what he was ordered to do - he did move slowly when told to crawl, but remained standing bolt upright as he slid across the terrain. Most unnerving. This was 3 missions into the Allied campaign.
Other than those minor issues, I've not had any other graphical weirdness whilst playing this fun game!