ok, this is going to be somewhat educational for many young collectors. Here it goes.
1. When new servers opened up for the first day, there were random items not locked down in time by the GMs and you can just literally pick it up. It was a race between the players to pick them up and the GMs locking them down.
2. There were some return home rares that each time you picked it up, few days later it spawns again and are able to be picked up again. The latest example would be the opening of the Kaldhun Dungeons with L-shaped rocks and magic doors. However, some of the older return home rares if you cross a specific server line with it, the item is stuck there and just falls on the ground or simply disappears when you cross a server line.
3. There are many items that were abudant for a period of time but discontinued and people called them rares. Bloody bandages, furs, and other similar types of items.
4. There are items that during certain events by GMs, Seers, and EMs that they forgot to lock down and players looted them and people are now calling them rares.