In .NET programming most common mistake is rethrowing an exception. So, how throw and throw ex differs?

throw ex overrides the original stacktrace and the point where exception is rethrown becomes the exception's origin while plain throw preserved the original stacktrace information and it must be used to re-throw a .NET exception. 

We are on a PoC regarding distributed application that leverages AppFabric Message Buffer. My colleague experienced that message buffer does not always behave like a queue. From what I understand message buffer messages are in FIFO manner.

What my colleague found out was that he stored 5 messages to the message buffer then moment later he stored his 6th message. A separate console application retrieving messages got 6th message first then the other five messages in order. 

We are still trying to recreate what happened.