I don't necessarily think that it's the "biggest" but I'll agree it's certainly one of the problems. I think I'd put "unclear specifications" as the biggest problem.

I've seen a lot of code delivered on time to meet the deadline that clearly was not ready but was delivered anyway. I've never the temerity to say, "I told you so," but I thought it. There's a balance to be had. Sometimes the deadline and the customer requirements and future maintainability cannot all be satisfied. Something has to give.