I bought a 10mm pistol silencer... modded it to my 10mm weathered pistol and now it glitches out hard. I equip the pistol and I can no longer see it and my entire screen tints a bright light, like someone threw a flash grenade at me or something... If I drop it, it appears as a giant red box with an explanation mark in it... Not too happy about this, how can I fix it?

... because someone screwed up and allowed a unique weapon to be modded? (It's not supposed to be possible).

Basically no. Load up an earlier save.

If it is a glitch with the modification to the 10mm weathered pistol then I would either:

1) Use a different gun

or

2) If the anomaly still exists then repair another 10mm pistol with your modded pistol, this should strip the gun of the modification, but you will lose your 10mm weathered pistol in the process.

I'm having this exact same issue. This and all the other problems makes me wonder why Obsidian and Bethesda made the conscious decision to allow so many glitches in the game.

I do not think any game Developer would ever make a "conscious" decision to allow glitches in their games, why would they? It is not like any game Developer intentionally allows glitches to be present when their game (in which they put countless hours into making it for our enjoyment) ships to the public just for kicks and giggles...

You have to understand, even with quality assurance and game testers, when 1+ million people play a game they are going to find bugs and glitches not regularly found in QA testing.

Besides, the game engine NV was designed with has been notorious for bugs, so it is not like it should be anything new..

Yeah, ALL mods to the weathered 10mm result in a big red "?" over your character. Lame.

There is absolutely no way they didn't come across any of the game breaking glitches this game launched with. Seems like they wanted to make the launch date and shipped knowing they could always update later.

You can find a mod here that allow you to add mods to your gun

http://www.newvegasnexus.com/downloads/file.php?id=34794