Minor potions.
 

Wasn't sure if it was a game mechanic or a spell so I erred on the side of caution in misc. I tested a Minor Null potion, Minor potion of cold, and a Minor potion of Heat. Each of the minor are buffing their specific resist by 5, then dropping to 1. So your stat would go up to say 70 from 65, then drop to 66. On zoning it appears to pop back up to the 5 it should be, so I don't know if it giving 5 or 1, or what. It may be just a client thing but it's is a bug I think.

This is a client-side bug. The developer that fixed this months ago said that the client is only improperly displaying the values. Your actual stats are still up by 5. I'm too lazy to find the original thread, but I guess this bug is back.