Year -- or rather, decade, century, and millennium -- numbering. We're only in the second year of the second decade of the twenty-first century, so the decade will end and the next start at the end, not the beginning, of 2030.

But this is perhaps more of a problem with numbers and zeroes, and ends and beginnings; people don't get that 10 is the last of the 00s, not the first of the 10s. The very first year was numbered 1, not 0, so that's not the problem. Or, it kind of is: People would be right, if the first year had been 0.