Ammunition. Can you ever have too much of it? I mean, unless you're on fire or trying to swim, I have to believe that the answer is no.
Still, after reading Tam's post on calibers that she keeps on hand, and Ordnancecorner's follow-on post, I count myself blessed to have been able to steamline my ammo bunker down to just 21 calibers that I need to keep in stock.
7.62x51mm (linked and loose)
I used to be obsessed, but I'm feeling much better now.
In the interest of full disclosure, I do plan to add three of four more in the near future: .45-70, .455 Webley, .577-450, and now .38/200 since someone I know recently bought a Webley MK IV, this re-igniting the arms race between the two of us. And if I can ever find a Boer Mauser that's not $1600, I'll start stocking 7mm Mauser too.
But like I said...I'm feeling much better now.