Questions about genetics, protein production, and strange unexpected phenotypes

[Mgellis]

Doing a bit of brainstorming for science fiction stories.  I'm trying to avoid any stupid blunders in my science that would make the story implausible to readers.  (I know there are some things we just don't know yet, but I'm trying to make a good faith effort.)

I'm thinking about unintended consequences of genetic engineering.  How plausible is it that if you added a gene (or genes) to a genome to produce a certain protein (or proteins) because it adds a certain health benefit that there might be unexpected and mostly harmless but very visible side-effects?  For example, if you modified human genes so they would produce proteins that help with bone growth so people would survive better in microgravity, and it works, but these people also grow little horns out of their heads or their fingernails are thicker than normal or their hair is some odd color or something like that.  Unexpected, and maybe a nuisance, but their bones are really healthy now so people decide to live with the side-effect.  If someone put that in a story, would people who understand genetic engineering say "yeah, that could happen...maybe not very often, but it could definitely happen" or would they roll their eyes and curse yet another idiot writer. 

From my very limited knowledge of genetics, this sounds like it's at least possible, but I may be missing something really basic.  As I understand it, there are something like 200 million proteins known to science (so I suppose there are genes that would cause them to be produced, although I imagine most of these are currently unknown).  I'm guessing at least some would have multiple effects on an organism, depending on how much was produced, the types of cells that encountered these proteins, etc.   

My apologies in advance if these are dumb questions.

By the way, have there been any real cases like this in experiments?  You know, things like a mouse that gets an extra protein-related gene so its metabolism will act in a certain way, and it does, but the baby mice are all purple or they crave apples over all other kinds of food.  Something that just was not expected.  (Bonus points if it something genuinely disturbing.)

Thanks in advance for considering these questions.