Why do my Dr. Brown's bottles keep leaking?

This is the single most common complaint in r/NewParents and r/HumansPumpingMilk threads about Dr. Brown's. The consensus fix: don't overfill past roughly 3.5 oz in the 4 oz size, make sure the collar isn't screwed on too tightly or too loosely, and remove the internal vent when using the travel cap.