In creative or survival, you can place coral fans onto the side of slabs. The fans will pop off momentarily because of random ticks, or if you update them they will also pop off.
How to Reproduce
1. Place an upper or lower slab
2. Aim at the side of the slab and place a coral fan
3. The fan will be placed, but will break off when you place another next to it.
Unsure if this affect other blocks, further testing would need to be done.