in my experience, false positives in early ventures (research projects, startups, project partnerships, whatever) are the kiss of death. If you need a group of people to do work at high capacity, bringing someone who does not 'fit' in when the group is young will probably destroy it. existing high-capacity people will become angry, there is less fault tolerance in the quality of the work and the rate at which it has to be done, etc.
it is usually the case that it is better to have 2 workers who click and are highly productive than 5 workers where 1 is not productive. so in the early days, failing closed seems very valuable.
when you are larger, there could be a higher tolerance. you could afford to hire someone and fire them a few months later if they do not work out, because the work that they are doing for you immediately is not as critical. however, there is also a certain culture that comes along with this and if you want to avoid that culture it seems like you still want to 'think lean' and part of that thinking could be to keep the amount of productive people high.
and it might also not go well with your current group members to say "yes, that hire was bad and we'll terminate them next week" for the fifth, sixth time in a quarter ...
