Other than not being able to claim any more IP addresses in a subnet, what other bad things happen when a subnet reaches 100% utilization, if anything?
A subnet (network) is really just a collection of contiguous addresses within a binary mask. It is simply a logical way to divide address block. If you run out of addresses in a network (subnet), then you are simply out of addresses in that network. Adding any more hosts would require reclaiming unused addresses, expanding the network (may not be possible), or adding a new network (requires a router to communicate between networks).