Thread title pretty much says it, when I enter a certain area of my SMP map all the modded blocks suddenly disappear as if they are missing a texture (eg ores that aren't standard) and you can see through them (and walls if there is a block behind them). I have RP and BC installed and their blocks also do the same thing, I doubt it is due to IC2 alone but I figured I would post just in case someone knew why this was happening. I also have Optifine installed, any ideas?
Try another version of Optifine, possibly even an older version if one is available for the version of MC you're playing.
Edit: Forgot to add because it was late when I posted that we had this problem before, and it was definitely that particular version of Optifine we were using at the time. We installed a previous version (HD_B6, I think?) and the problem fixed itself. Now we're using HD_D6 without issue.
If you use the Magic Launcher, you can test out different versions of Optifine without constantly having to redo your minecraft.jar.