Of course we've run into problems from time to time. No one goes from nothing to foursquare's level of success without running into some bumps along the way.
> were they serious? did someone lose money?
> it would answer whether to use an eventually consistent db
MongoDB actually isn't really an eventually consistent datastore. It doesn't (for example) allow writes to multiple nodes across a network partition and then have a mechanism for resolving conflicts.
You had 11 hours downtime and didn't lose money?
What about opportunity cost? Reputation?
Now you have to share your secret :)
(I guess, if you weren't profitable, you had nothing to lose?)
Or perhaps investor pressure to close ranks ;)