Yes, I've noticed the same. I've had to stop behaviour loading on any pages I'm using Mootools.
I think the error is:
Behavior is not defined.
Must be a conflict or some sort, but I havn't delved deeper. It's best to avoid Mootools unless Prototype can't help you, as prototype is packaged with Silverstripe, and is totally incompatible with Mootools. (last I heard)
Prototype is used throughout the front end of the site - page comments, login, basically anywhere theres a form!.
One of the things coming up is SS is moving to a jQuery JS library and away from prototype. I know quite a few of us in recent projects have spent some time stripping Behaviour and prototype out of the various front end components to provide us with a clean slate. Some of this work will be part of trunk soonish I guess unless its already in there!
If you are trying to use mootools you will have to dig round and round digging out the old Behavior JS as it will pop up like a weed!