Bukkit

It appears when you start to mine a block, without the proper tool in hand, the switcher switches as you are mining. If the tool is able to mine the block before it goes back to a complete non mined state (diamond and gold tools seem to be able to do this) it causes the block to be both destroyed and existing, and causes the client to crash (out of memory).

Althought it happened frequently before, It only happens on occasion now.

I am going to try to replicate this, although I hadn't noticed anything like this in my testing. This mod doesn't do any canceling of block breaking, so there could be a weird clash with some permission checking (most of my testing has been as op).

Another possibility is the client is getting confused if a switch occurs at the same time as the block breaks (more likely with gold and diamond tools).