When I went into the Nether I discovered there were absolutely no Nether Ores in sight. I loaded a new single player world and my current single player world to see if the ores were there, they weren't. They are not on my multiplayer server either, but I can still give myself the ores and place them with no errors.
I am also getting this after the "loading NEI" appears in the server.
"2013-03-17 12:24:17 [iNFO] [sTDOUT] Loading NEI
2013-03-17 12:24:18 [iNFO] [ForgeModLoader] Unloading dimension 1
2013-03-17 12:24:18 [iNFO] [ForgeModLoader] Unloading dimension -23
2013-03-17 12:24:27 [sEVERE] [ForgeModLoader] Detected leaking worlds in memory.
There are 2 worlds that appear to be persisting. A mod is likely caching the wo
rld incorrectly
2013-03-17 12:24:27 [sEVERE] [ForgeModLoader] The world 1989fc9 (world) has leak
ed."