I would like to hear what kind of design decisions you took and how did they backfire. Because of a bad design decision, I ended up having to support that bad decision forever (I also had a part in it). This made me realize that one single design mistake can haunt you forever. I want to learn from the more experienced people what kind of blunders have they experienced and what did they learn from them.

I'm sure this will be a lot of help to other programmers by helping them to not repeat those decisions.

Thanks for sharing your experience.