In The Late Philip J. Fry (S06E07), the professor invents a forward time machine which causes an adventure through future years in order to find a backward time machine (after the professor overshoots). After the universe decays and the second big bang, the professor stops to kill Hitler before continuing on. He then overshoots again and the second time around he attempts to shoot Hitler in a "drive-by" (I don't know what you'd call passing by in time) which misses and hits Eleanor Roosevelt.
Both times, this event doesn't appear to have changed anything at all in the future. The only noticeable difference is that the final world appears to be about ten feet lower than previous. Normally, in pretty much any other show that deals with time travel, we're warned not to change anything in the past as it can have unintended consequences on the future. Why don't these changes in the past affect the future at all? Why does the Professor, who would normally be warning us about dangers such as these, choose to kill (and try to kill again) Hitler?