Twice actually. The first time it someone who I grew up and was close friends throughout my schooling and higher studies. It started out well enough. They offered me sweat equity upfront (no cliffs or vesting) so that showed a lot of trust. As time went on, him and his partner began spooling off different companies with no intimation and conflicts of interests. They even diluted my stake without informing. I waited till I was 4 years complete, asked them to hike up my equity stake by a few points as the business while running was not making a load of cash and the salary I was drawing was peanuts compared to the market offers I was saying no to. They didn't respond on that request for a year. Put in my papers. As to how our childhood friendship fared. He had a child recently and I found out via common friends :-)

After a stint in another startup, have joined another friend at his startup. It is hard but there is much better transparency. Only time will tell though if the friendship will survive the stint (I hope it does).