Everybody loves bus bunching.
Or more realistically, everybody loves to complain about bus bunching - when two or more buses (usually on the same line) should be evenly spaced out, but are right behind one another. Here around UC Berkeley AC Transit's 51b in the line most people complain about bunching (they're working on it!), but every transit systems has its own problem line(s).
Earlier this week WBEZ's Curious City examines bus bunching in Chicago. They provide an easy to understand animation that demonstrates how minor service delays cascade to bus bunching. Bookmark it to share with your friends next time they lament about the topic.
The topic is also beloved by transit researchers, particularly at ITS Berkeley. From systematic analysis of why bunching occurs to ways to solve the problem. And as always, you can find more research on bus bunching at TRID.