when placing a rail in creative mode, it breaks blocks near it (fine, thats normal, its meant to) but if you place it right next to the IR machines like the melter and roller, it breaks the multiblock, but the multiblock doesnt realise so it works as normal but doesn't render until you interact with it then you crash.

so if you form a melter, form it it seems fine. place a rail in creative with "block breaking:true" next to it and it breaks the blocks that it should however the multiblock doesnt sem to realise and continues to function (although the rendering does dissapear so its invisible). upon interacting with the broken multiblock more it will crash you because then it realises "hey, i am a multiblock but im broken, but i am formed even though im not?"

update: relogging does not fix the error, however if you manually break enough of these broken invisible blocks it does eventually fix itself (not one block, takes several)