Two thoughts. First, if your startup requires people to give up nights and weekends and you need to ask potential candidates how they will mitigate that situation with their family life, then you need to rethink how your business and software development run. Work life balance is exceptionally important and should not be ruined for the sake of building a business. I realize you didn't bring up the question about giving up nights and weekends. But since the poster above you did, I thought it important that I bring up that requirements like that are absolutely absurd. If you regularly take away peoples personal time for your business, rethink how you run a business.

Second, on to your question. I have hired quite a lot of people and occasionally they are amazing in interviews and on paper and don't work out in real world situations. Over time you get better at spotting these people but it is a risk you take. As with many things, interviewing is a skill and some people are quite good at it. Often times it is not a technical or job skill mismatch, it is a personality or a team fit match. That can take weeks or months to uncover. The best thing to do is to have a clear exit strategy for each person you bring in if they don't seem to work out.

Yes, I had it happen that people are great on paper and in the interview and did not work out. In each case it was a sales hire. The people in question were excellent sales reps. They knew how to sell themselves as well. However, one of the reasons I hired them for (a big and super relevant rolodex of C-Level contacts in our industry) turned out to be the main reason they totally failed. These guys valued their contacts overall. They would never hard sell any of their contact if they had even the slightest doubt that the product would work 100% as expected. Well, we all know how startup products get build and how they often fail before they excel. So these top notch sales guys would keep on setting up all these great meetings with big names but never close. First they tell you that closing a deal with a blue chip company takes forever (true, they often do take forever). So they could ride along for quite some time without getting caught. Then when pushed, they would make up fair excuses, why their contact would not buy our stuff. This happened to me in my early days as a GM and it took me about a year and a half to see through this.

Now I never hire a sales person based on his rolodex anymore. I find out if the sales person is truly passionate about the product.

Also, on that note, stay away from the sales guy who tells you that he will sell anything to anyone. My best sales hires in the past years have always been people who think that they are not sales people at all :)