It seems that this is a serious but less talked about problem to watch out for. Anyone have insight into how to avoid one of the less visible mistakes beyond the obvious know your co-founders well and set expectations upfront?
If you don't know anyone like that, you can manufacture them. Find good people and try working with them on projects that are not the startup. College and (especially) grad school tend to create these situations, but you can also make them happen deliberately, e.g. by collaborating on some open source project.
The chances of us interviewing a team of founders who'd built some fairly well regarded open source software together would be close to 100%.
Teams thrown together for the sole purpose of YC seem likely to have poor records. But YC is a very, very special case. Bootstrapping takes time and effort that either deepens relationships or destroys them. I don't see a middle ground there, actually.
Have you ever looked at how long a team has known each other as a predictor of success/failure? It seems your philosophy assumes more is better. Is there any data to support that belief? What's the record for spouses?
More importantly, I don't think pg's saying that length of relationship is a deciding factor. Rather, he's highlighting that you need to know you work well with your cofounders and the relationship – new or old – won't fall apart under stress and trial.
When PG says to try projects before a jumping into a company he’s saying: you should date your girl for a while before moving in together.