They come from the cost savings made by a pointy-hair telling that smart aleck code monkey that bothered actually reading the spec that his concerns aren't going to delay the ship date or go into the budget for developer time -- but you know maybe in six months (every six months, it'll be six months out) we can totally revisit that.

Then it ends up in production in too many customer sites with IT that avoid updating stuff (because the updates frequently break things, which is a pain) that it's too late to fix it now. The concerned forward-thinking developer moves on to (slightly) greener pastures to rinse and repeat and the project gets handed to an off-shore team who don't comprehend the difference between current behavior and specified behavior of a product - yet another bold cost-saving measure for pointy-hair to use to argue for his bonus.

Not spending time thinking about handling those, not debugging anything related to them, etc. The code might look almost the same, but it's always a cost of how to get there. And the code in these middleboxes not rarely smells.