This one totally screwed my last game:

The United Planets law "Tax on starbases located in someone else's sector" was voted in. It was supposed to tax a race 8bc for each starbase they own located on a tile that is culturally controlled by another race

BUG SYMPTOMS:
1) On the United Planets screen, the law is listed as costing 81 bc per base per week instead of 8 bc!!! HOLY CRAP!
2) In my last game, I controlled more than half the galaxy in my end game. I had NO bases in enemy territory. Somehow, the law is reversed, and I was paying out for enemy bases that were in MY territory! My per-turn expenses listed as "United Planets Starbase Tax" totaled 2376. That was more than ALL of my other expenses put together!!!

In the late game (when there are a lot of starbases out there), this destroys your economy. Only solution is to break all your alliances and blow up all the starbases.