How do people get bugs as weird as this, it just boggles my mind. About the only thing I can recommend is doing a reset + repair in Assist. Short of that, it might be a memory corruption issue on your computer.
To be honest, it's not that rare of a bug. You get it by playing hours and hours
I get it every now and then. Not really sure what causes it - I only know it happens a lot when I put the 203 away without shooting any 203s - and the only thing I know that will remove it is jumping and throwing away the gun (as was said already).