Quite often, legislators include what's known as a "severability clause" in their bills. These are meant to protect the bulk of a law in the event that a small portion of it is determined to be unconstitutional. That small portion must go, or be changed, but pretty much everything else is allowed to stand.
In a sin of omission, Democrats left such a clause out of the health care law, and now the plaintiffs in at least one of the cases against it want the court to take an axe to the whole thing if the judge decides that the individual mandate provision is unconstitutional.
This isn’t the first time Democrats have made a small error with big consequences—you may recall the food safety bill problem:
In what amounts to an epic constitutionality #fail, Senate Democrats may have blown their chances to see their food safety bill signed into law.
The U.S. constitution requires that any revenue-raising bill must originate in the House of Representatives. To honor this provision, the Senate often finds a discarded old House bill, strips it bare, and uses it as a "shell" and passes it back to the House.
They somehow forgot to do that this time.
That version of the bill went down burning; they had to pass another version of the food safety bill, stripped of its funding provisions…which are conveniently up to review by a GOP Congress and/or the possibility of “industry cooperation,” always an ominous phrase from a regulator.
Sloppiness—it can hurt. The odd thing is that I can't quite remember such errors in previous Congresses, though maybe my memory is faulty or I didn't notice such things in the first place. Common sense is that some errors of this sort should have occurred, but I just can't remember; nevertheless, very frustrating.