This is an interesting solution, from http://www.blacet.com/tech.html, to provide protection against reverse connection of power supply as well as overcurrent protection caused by any kind of short circuit:
Power input protection and filtering is highly recommended for all modules! See below.
PS1 and PS2 are polyfuse resettable fuses available in various mA ratings. A typical module with a 50 mA current draw would use 100mA versions.
A power supply reversal causes the diodes to conduct, creating a short circuit. That condition or a short on the PCB causes the polyfuses to heat up and go into current limiting.