This happened to me too on a Bukkit server that I often play on... I used a drinking potion of Fire Resistance for 8:00 and it ran out at about 2-3 left. This has happened multiple times. The first time iIlost 8 diamonds, but the second time I was prepared 😛
Also, the time doesn't just seem to be reduced. It seems to be random. This also happened to my friend a couple of times.