On computer programming
It's hard to make analogies about being a computer programmer because I honestly believe it is quite different to other professions in which practitioners tend to be held accountable for their actions. With programming, you are in charge of not only the Platonic ideal but also the real-world implementation of any solution. The implementation always sucks, but nonetheless people don't just put up with it but actually seem to think that what you have produced is innovative and life-enhancing.

It's sort of like you're a combination architect / builder, but you're a really terrible one. Someone asks you to build a new bathroom for their house, so you do, but despite all your careful planning the finished product is just shite. For example, the shower wall is made out of soap, because you thought it would be convenient -- now nobody needs to buy soap, because they can just rub up and down against the wall for a bit and then hose themselves off (you installed a hose instead of a shower head for greater flexibility).

After a few days, though, the drawbacks become obvious, because rats start eating the soap. Every morning the owner, Susan (let's say), enters the bathroom, chases away the rats, rubs up against the wall and hoses herself off. You know you should fix the rat problem but it's too expensive to replace the wall, so you just mix rat poison in with the soap. Now Susan's boyfriend is complaining that she smells of zinc phosphide and there's a growing pile of dead rats in the bath. No problem though, because you hired a man to clean them out. Every week, on Thursday, sometime between 7am and 3pm, a big, burly man shows up with a spade, lets himself in, shovels up all the rats, puts boot prints on the carpet for no obvious reason, dumps the rats in the front garden, and leaves.

Of course, nobody can use the bathroom while he's in there, so anyone who wants a shower on Thursdays (and many people do) washes in the kitchen sink instead. You do a deal with a builder and soon your soap wall / kitchen-sink shower houses are everywhere. After a couple of months owner's clubs pop up where residents talk about how great it is to wash yourself in the sink because then you can get all your daily chores done from the one spot.

EDIT: Sorry about the comment screening. It's just because of idiot spammers.