I just ran into a problem, when I manually configured the packet buffer too small in the gnrc_networking example. In fact, I copied the size from the gnrc_minimal example (512 bytes). Testing this on the iotlab testbed with ~350 nodes (Neighbor cache size set to 100, FIB size to 350) I ran into several crashes. Nodes were dying with hard faults after some time.

(On the pro side: with the default packet buffer size, I was able to transmit a UDP datagram from the one end of the testbed to the other without problems. :))