It's hard to say as you've not provided much information.
What I suspect has happened is that you've created a technology loop somehow, where two advances are prerequisites for eachother. For instance making Physics a prerequisite for Magnetism, and Magnetism a prerequisite for Physics.
Alternately, did you delete lines in rules.txt or amend 'Future Tech' in any way?
1) Have you accidentally deleted any of the silly periods or commas? That's a nasty booboo.
2) Have you changed the three letter code behind the semicolon and then made a unit with that prerequisite? That code is purely a comment and cannot be edited.