Before you board the bus, you're given an envelope containing tickets. Check the expiry date at the bottom on the back of your tickets before boarding.
When you first get to Fountainebleau you pick up an audio guide. I was annoyed to discover that my ticket did not include the audio guide even though Paris City's promo states that audio guides would be provided for both.
However, after paying six Euros for the audio guide, THE WORST PART was that when I scanned my ticket, my entry was barred. Luckily an attendant was available and showed me that my ticket had expired in 2019!
Given my crestfallen expression and panic as what I was supposed to do now, she graciously let me in, but strongly advised me "Do not use this company again!"