The PageMethods Returned an System.InvalidOperationException while I was returning a dataset. It was interesting to note that the probem only occured whin i was trying to return more data. 

I was using the code to populate the city when a user selects a state from a drop down list. Ideally a cascading country , state, city cascading drop down list. 

I figured out that the data has problem only if the number of cities in the state was higher. 

Probem was solved by removing maxJsonLength from the jsonSerialization  tag. 

      <jsonSerialization maxJsonLength="500">


Please donot forget to uncomment the jasonserialiation in case you want to return a dataset in pagemethods