Collections longer than 100 videos
It is possible to add more than 100 videos to a Collection, which is good. However, it is NOT possible to view videos past the 100th video in your Collections manager. Also, if you are watching the Collection, the videos stop playing upon finishing the 100th. The only location you can view past the 100th video is in your Videos tab on your channel.
I have a long video series myself that needs this. You also can't reorganize videos order on that collection easily past 100. You have to remove the latest entries and re-add them in the order you want the videos to be in.