In current design a tailable cursor on an empty capped collection will be released immediately without waiting.

db.createCollection("testz",{capped:true,size:100000})
db.testz.find().addOption(2).addOption(32) //doesn't wait
db.testz.insert({x:1})
db.testz.find().addOption(2).addOption(32) //waits

This behavior is very annoying for users who are creating tailable cursors in their apps, as they have to handle this case in some way to avoid high-intensive polling.