Would it be genetically possible for an error of sorts to occur?

"Flaw" and "error" are both loaded terms revealing your own bias. There's no room for value judgments in biology. Certainly not in genetics.

Sexual attraction could be affected by genetic polymorphisms, IF it happens to be a genetically heritable trait. The facts are still unclear. The best understanding we have right now is that sexual orientation is the result of a combination of genetic and environmental factors, with the genetic part almost certainly involving multiple genes. If that turns out to be true, probably the most you could say is that certain genetic polymorphisms tend to have an influence on a person's sexual orientation.

Edit: No, it's not OK. You're assuming that homosexuality automatically leads to reduced fitness, which may or may not be true (leaving aside the underlying assumption that life has a "goal" of reproduction). Firstly, there are many examples of organisms engaging in homosexual activity and still reproducing, including in humans. Secondly, even non-reproducing individuals can contribute to the fitness of their gene pool. See social insects, for example. In humans, there's a theory with quite strong support that non-reproducing members of a given society contribute greatly to the success of that society through social and economic factors, allowing that society to outcompete its neighbors.

So the assumption you made - that homosexuality is the same as reduced fitness - needs to be proven, not simply assumed.

Well, technically, yes. A lot of hormones affect how we behave towards the opposite sex, or make the opposite sex attracted to us. So if those are absent or have some kind of error they most likely would affect our sexual urges for example, or our need to mingle with the people of opposite sex. Also genes do affect the brain and can result is some kind of defects. Psychopaths are believed to behave in such way due to certain abnormalities in their brain (basically they can't really control it). So maybe we are attracted to the same sex for the same reason, because our brain surpasses the need to reproduce (which is instinctive). I don't know. I'm not a scientist so these are just speculations. It could be that people just want to bond to someone and if it happens to be of the same sex, in this day and time, it doesn't really matter that much, so they go for it.

It is in no way a biological error. You can't inherit that from anyone. It is not dominant, recessive or anything else. Physical attraction doesn't come from an allele your parents gave to you. It's just your mental mindset based on your natural responses to the world. It has to do with the way you were meant to develop. It does have a little to do with genetics but it's not a flaw or disease, and you can't get it from anyone. I happen to be bisexual. You may have your own opinions but I think we can both agree you are attracted to your own gender, and let me be the one to tell you, no matter what you do, what you try, where you go, there is no changing that. You're not going to after some therapy or magical remedy, magically start liking the opposite gender. You have to learn to accept yourself despite your own biases, or be miserable.

of course it is possible that a mutation causes same-sex attraction.

In humans the biological causes of same-sex attraction aren't well understood, but better for males than for females.

Most likely it is not a single reason, but a lot of different ones.

but in fruit flies a single gene seemed to shift behavior from female to male:

http://www.nytimes.com/2005/06/03/science/03cell.h...

http://www.sdbonline.org/fly/dbzhnsky/frutles1.htm