[jacalata]

You build a house for me for $X. Two years later, I notice that if you hang three towels on the bathroom door on a humid day while the living room light was on, the door didn't close properly. Would you be happy if I came to you and demanded that you debug and fix this problem for free? Didn't I pay for a working house the first time?

I very much do not believe that you paid me enough for me to guarantee that my product would work in all circumstances forever. If you did, then by golly I'm yours for life buddy!

[cynix]

If the problem is due to my poor worksmanship and it has always been there since the very beginning (even though you may not have noticed it for 2 years), I'd gladly fix it for free. Of course, with a bathroom door you'd have to prove it's not due to wear and tear, but this doesn't apply to software - the bits in your binary don't change with time.