crash when zombie pigmen falls onto rail track

Playing on a friend's private server in the nether, and building a covered rail through an open network.

I'm building a roof over the tracks with cobblestone half tiles, 3-tiles above ground (so 3.5 tiles drop from the rooftop as it's placed on the bottom half). When a wandering Z-pigman (neutral) decides to drop from the roof under construction onto the railway, this caused an unrecoverable crash and forced me to use the power button. The frozen screens shows the pigman just landing, and I hear it's landing (but not a squeal from falling more than 3 blocks) just before the freeze.