I've been thinking about this topic for a bit -- this notion of surprise, and how what is unplanned can be a good thing in terms of community.
My first example would be at MindCamp and the lack of internet access. Though it was billed and planned as having a mesh network, things didn't work out as planned at first. Partly due to tech, partly due to lack of nodes in the mesh network, there just wasn't any connection at first. Though this did seem to put a hamper on things, I would argue that this fostered community because 1) people were forced to focus on the topics at hand 2) people had to interact with one another 3) you had an instant ice breaker of "gee, it sucks that there's no internet" 3a) shared, common experience. Apparently, others also believed the lack of access to the internet to be a good thing as evidenced by some feedback here on the wiki.
Another example of how the unexpected can be a good thing would be what happens in Seattle when it snows. First, it rarely snows, and if it does, it rarely sticks around. So it is not that surprising that people in this city get all "weird" when it comes to snow. For days the top news story was the snow. This overall weirdness though, is somewhat unique though. At work last week when it snowed, the focus of everyone in the office was elsewhere. People worried about how to get home, some had to get their kids from school, others looked in awe at the big fluffy flakes falling from the sky. Regardless of how individuals reacted, there was this overall giddiness in the office. Likewise, it seemed that there was this sense of wonder for all experiencing the snow. Just a few years ago when it did snow heavily and stay, the city of Seattle literally shut down. Hills turned into ski slopes. Neighborhood restaurants never looked so packed. The place down the corner from me turned into a ski chalet, offering free hot chocolate to those braving the weather.
In both scenarios, this notion of surprise and shared experience seems key in terms of bringing people together. It seems to shake people momentarily from their day to day routine, and we are all then able to look at the world with a sense of wonder, possibility and play.