The game crashes every time I try to place a crimson slab in the lower half of a block that already contains a red netherbrick slab in the top half of that block. Doing it the other way around does not result in a crash (crimson on top, red netherbrick on bottom).
Thank you for your report!
We're actually already tracking this issue at MCPE-73435, so I will resolve and link this ticket as a duplicate.
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