I heard about two cases when MongoDB failed doing The Most Important Thing - storing data. No one really care about autosharding, no migrations etc. if you can't store the data. Due to some replication issue data was inconsistent.
But can't this happen to any db system? Mongo is pretty new and I'm not surprised things like this happens from time to time until the kinks are worked out. The new version of Mongo looks pretty good as well.
