Minecraft wiki entry for Slab, associated text: Any light directed through a slab does not affect any block's light values north of the source.
Whereas 1×1 dents in a flat floor using slabs are darkened as one would expect, 1×1 dents created using 2×1 or 2×2 stairs darken less, due to having fewer surrounding solid blocks.[11]
When water or lava are on top of an upside-down slab, the water dripping particles appear in midair below the slab instead of from the slab itself, this was fixed in Java Edition 1.13-pre7,[12] but will not be fixed in Bedrock Edition.[13]
When a TNT placed on a slab explodes, its blast radius is greatly reduced. ( In some cases, only the slab is destroyed. )
As with the previous trivia, end crystals and creeper explosions will also be weakened if the explosion happens on the slab.