User blog comment:DEathgod65/Battlefield 3 Patch Notes/@comment-112.206.102.17-20120331011609/@comment-1967485-20120331155046

Well before the 1.04 patch, if you fully finished all unlocks ((so you can now use the M4A1 on the RU and AKS-74U on the US)) and you picked the opposite faction weapons, it would switch to the opposite of that weapon on the next faction. ((It's a little confusing, but a dumbed down version: If you use aks-74u on US, class changes to M4A1 on RU)).

Now that doesn't happen, so no more annoying weapons changes every match! But I guess the pistols are a drawback/bug of it.