Are you sure it wasn't rescored from 3 defensive TDs down to 2? One of my fantasy leagues is through Yahoo, and last night they had Chigaco having 3 defensive TDs, good for 26 fantasy points. Today they adjusted it down to 2 defensive TDs, 20 fantasy points. Both defensive touchdowns came on punts- one was blocked and taken back for a TD, the other was a fumbled return, taken back for a TD. Here's a link to the box score...
NFL - Minnesota Vikings/Chicago Bears Box Score Sunday October 19, 2008 - Yahoo! Sports