When the client is heavily modded, chunk errors regularly happen in huge groups, they are usually 1-2 chunks by 7-21 chunks for me. Even though the client is modded, it is not a problem with the mods. I have tried it with multiple groups of mods, none of them having any mods that any other groups had. last note, I am experiencing this on singleplayer. I do not know about multiplayer, as I do not play modded smp.

IGNORE THE 1.5 TAG! THIS OCCURS IN 1.4.7 AND EARLIER, I HAVE NOT TESTED BEYOND THAT!