“Sometimes your investors know best. Sometimes they don’t”

Sensationalist title. I think of "calling bullshit" as pointing out dishonesty, not just saying that you disagree. For example, if a friend said he won $50,000 in Vegas, I might say "I call bullshit...show me a receipt!"

In this case, PG made a suggestion about the name and logo. If you tell him you're going to stick with the name Reddit and the alien logo, it's not "calling bullshit" it's respectfully disagreeing.

Kind of weird that this even needs to be said. Do most people really have that strong of a fetish for authority? Experts/advisors have lots of experience in general, but you have more experience with your specific situation. Someone who thinks about your situation for 5 minutes/day is not going to have as nuanced of an understanding as you do, even if they are able to bring a lot of experience and knowledge to bear on the problem for those 5 minutes.

None of this is specific to Paul Graham. This is general advice to anyone receiving advice from someone with less expertise in a certain area.

TL;DR When someone gives you advice about something they know less about than you, take it with a pinch of salt.