I get it on occasion. The server lost your session identifier. The identifier is what is stored locally on your computer and is sent to the server so the server knows who you are. When you submit again, after the invalid session message, behind the scenes, a new session is created and then you can post again.
It is a bit scary if you've typed a lot and you fear losing all of the text you've labored over for the last whatever minutes.
If I write a lot and it has taken me a while to write it, I copy my text before I submit it.
Your session can be lost if you take too long to type your post. Say you are just slow typing, slow thinking, slow to edit, slow to get your message perfect or you might not be slow.... the phone could have rang while you were in the middle of typing your post so you let it sit, came back later to finish it (it that time, while you were gone, your session identifier on the server expired and so was deleted).
Given the scenario, Prolijo probably gets the invalid session message ALL THE TIME!!!

Prolijo has probably learned to type his messages in Word or Wordpad so this won't happen to him..... anymore. LOL!
Oh, sometimes the server can reestablish the session without you knowing. It actually depends on the browser you are using and if you've asked the browser to store your ID and PW.