It's something that has been mentioned so we'll definitely look into it. One suggested fix is to reduce your settings down from Ultra if you've got them up that high and see if that helps.
1. Is this on all maps or one in particular?
2. Did the invisible gun still function? Were you able to shoot? Did it re-appear if you cycled through again?
Yikes, that cannot be comfortable! Were you doing anything in particular to cause this? Perhaps switching between first and third person whilst reloading?
Thanks for reporting the issue. I've just tried it in skirmish and found that pressing G when the grenade isn't the selected weapon it doesn't throw it. If you then change to the grenade it automatically throws it, like it's been queued up.
Have you noticed a similar thing?
It could be a DirectX issue, make sure you have the latest version of DirectX installed. It's also worth making sure your graphics drivers are up to date too.
If you haven't already, make sure your graphics drivers are up to date.
Secondly if you find the logs folder in the Renegade X installation, check the Launch.log for any errors. (Mines located here: C:\Program Files (x86)\Renegade X\UDKGame\Logs)