1. Merchants/Vendors randomly stop updating their wares.
I have seen many posts about this both on these forums, and at the GameFAQs FO:NV board, mostly in regards to the Silver Rush merchant. I personally have had Cliff Brisco of Novac, Chet of Goodsprings and Lupe of the Grub N' Gulp stop updating their inventory.
2. Merchants/Vendors that reach 30,000 caps will make sold goods disappear with no caps awarded and will no longer update their inventory.
This one is documented on the wiki, and I personally have been affected by this with Knight Torres, who unfortunately reaches 30k caps easily when selling you T-51B and high end weapons.
