before doing anything most times it's a good idea to save the game with a save as new name thus keeping the original game as it is and while trying different things in game to try fix an issue sometimes the combo rabbit holes can cause an issue and replacing them with single rabbit holes can help and one player had an issue with a single rabbit hole if I remember right the sim had an opportunity at the science building bit couldn't enter the building the solution that worked for them was to go to edit town then grab the building with the hand tool move it back a wee bit then drop it lemme check crins blog for issues with combo rabbit holes BUT having said that the chances are you may still loose the opportunity and have to get it to roll around again
will edit this post with link/s
edit to add link and also you can wait for a reply from crin as she may have a better solution than I 🙂 and I may learn something new too 🙂
http://sims3.crinrict.com/en/2012/10/opportunities-cant-be-completed.html