the /spreadplayers command spreads people onto signs, making it possible to spawn above a high abyss.

how to reproduce:

1. build a platform like in the screenshot attached
2. run the spreadplayers command

 /spreadplayers <x> <z> 0 2 false @p 

with the <x> and <z> from the middle sign
3. observe that you will fall through the signs about 3/5 of the times (because 2 of the 5 rows are solid blocks).

this example is built this way to show that it also fails if there are possible blocks to get spread to. of course if you only have signs, it will also put you on the signs.