%Floating gravity blocks
Using the excavation tool on some group of blocks which has a gravity block such as sand above it, will leave the sand floating.

%The crash
Updating the bottom sand using a torch while holding down the excavate button or with excavation toggled on will crash the game

%Setup
Here's the setup of 3 blocks stacked on top of eachother with S for sand and B for block. You can have a tower of sand on top of the one required for the crash if you want.

S <- step 2. update this one using a torch while excavate is active.
B
B <- step 1. excavate this one.

%Possible solution
I don't know if you need to do anything about the crash. Just make sure to at least update the blocks above the ones being mined to prevent floating gravity blocks. I'm not sure if updating anything else around the mined blocks is necessary. I realize that sand is supposed to stay floating in case the excavated block is the sand itself, but I guess this check should only apply if the block being excavated isn't the sand? Then there's if you have two gravity blocks on top of eachother. Maybe just update other types of gravity blocks, excluding its own type