When we were voting on new members, it was a silent vote with unanimous consent required. Any vote could be retaken at the order of the master. This means that if someone is eligible and should be a mason, and the vote comes up with a negative vote, the master can say a few words suggesting people reconsider and take the vote up again.
There was only once that I saw someone not pass the first ballot to become a mason. The master said that we should reconsider, and most of the brothers in the room chose not to vote the second time (as a symbol they were not the negative vote), and many of those who did showed their ballot before casting it. The second vote passed.
This as a process indicates there's plenty of room for abuse: any single master mason can anonymously vote to exclude someone for any reason they want. It's up to the folks around them to set them square if they are out of line. I can easily see an lodge existing where there wouldn't be any members willing to speak up in someone's defense.
It's my belief that none of what I've posted here is exposing any of the secrets of a master mason; but if any fraternal members disagree I'll happily edit and delete this post, and I don't intend to expose anything; but instead demonstrate that masonry (as well as, I'd assume, most other 'secret' societies), is as good as the members in the lodge.