Dropped items don't seem to interact with slabs correctly in the most recent 1.2 beta (I don't know if this is an issue under older betas).
The first problem I noticed is that items no longer go under slabs (see screenshot). The items used to slide under the slab and come out the other side, now they just pile up against the slab.
Another manifestation of what looks like the same problem is that now when an item drops on a bottom slab it will try to slide off instead if staying in place where it lands. Items in general seem to be repelled from slabs now.