riddle - I am a computer’s nightmare, but a programmer’s best friend

I am a computer’s nightmare, but a programmer’s best friend.

I am both the cause of a problem and the source of its resolution.

What am I?

Answer

How about

Stack Overflow (the error and stackoverflow.com at the same time).

I am a computer’s nightmare,

If I was a computer, I wouldn't like these errors at all. Too much data to keep track of.

but a programmer’s best friend

stackoverflow.com is one of the best places to go to if you get stuck while programming, according to this

I am both the cause of a problem

If you get a stack overflow error, you have a problem. There is not much you can do once this error happens.

and the source of its resolution.

You might be able to find help on stackoverflow.com, for example here.