At the end of Operation Cast Lead, Hamas came out with a press release that claimed that only 48 "mujahadeen" had been killed during the fighting, while they estimated that they killed some 80 Zionist fighters.
Since then, they have kept adding more and more Gaza "martyrs" who were members of the Al Qassam Brigades to their website. At last count, Hamas has listed out the biographies of no less than 305 Al Qassam terrorists with details of their careers as fighters. (188 of them were listed as "civilian" by the PHCR.)
So in the end they understated their real casualties by an order of magnitude, and they also overstated Israel's casualties by an order of magnitude.
I'm sure that they are equally trustworthy in other areas.