feature creep
Feature creep or scope creep is what happens when a project (usually software) spirals out of control and starts to have features added to it that were not intended, were not in the original design, or were not paid for. Feature creep usually results in bad times for the builders of the project as well as the company they work for.

Feature creep is generally a result of poor management or a bad idea, in which nobody had the balls to say "no" to either the bad design in the first place, or to the guy who wanted to add the features (which can be as often one of the people building it as one of the people paying for it).

Feature creep has made its way into the public arena as mission creep, to describe a political or military adventure that has gone out of control in the same way, for the same reasons.
I would have been out partying this weekend, but thanks to feature creep, I was working all weekend instead.