I've never had this happen. When it happens, consult the team. The truth is we all played Sims 3 for 3 years with only 5 traits so I'm not sure it matters in the long run. But since it's hypothetical I won't give an answer. When it comes into practice and you're at a screen that gives you a trait that is obviously incompatible PM me, yahoo IM me, come post here and I'll give a verdict.
I think as soon as I get a chance to sit at the computer I'll do some testing beacuse as I said - I've never seen it. I have seen the game not give traits a lot though. So was it the old not give a trait glitch or because it was incompatible. I won't know until it's been tested.