if the tournament is run using a "moving button" rule, as opposed to a "dead button" rule, you may actually have 2 big blinds posted.
the first way it would happen is, if in the previous hand, the player in small blind position gets knocked out of the tournament, the next hand the button still moves, but an extra spot so whoever was the big blind is now the small blind and the button, leaving 2 players to post big blind. the hand after that, it would be small, small, big blind, then the hand after that blinds would be back to normal.
another scenario is if the big blind is knocked out of the tournament, then the following hand would just be 2 big blinds. and the hand after that, small small big, then back to normal.
with a dead button rule, the button would rest in an empty seat's spot for one or more hands until the blinds caught up to it. moving button makes the button move one spot every hand, with blinds catching up eventually.
for illustrations, check out HPT's explanations of the different rules
http://www.homepokertourney.com/button.htm