AD, OD, tips/gotchas???

I searched the forum and didn't see any topics directly related to this, but I was wondering if you guys could share any hard learned tips/gotchas you discovered with regards to using AD, and/or OD, magic triangle, etc. Things you did at first that ended up biting you later, wasted time, required a relatively major outage to fix, possibly required touching all of the clients, etc. Thanks!