I had noticed some weirdness when you create projects, it's pretty easy to make My: Awesome, (Project), resulting in a #my:_awesome,_(project) hashtag. This is strange, and seems harmless, until you try to do something like update a revision which has projects attached with weird hashtags, and arcanist bails on you:
user@machine:~/thing$ arc diff Exception Error parsing field "Projects": The objects you have listed include objects which do not exist (#perception:_odtac_(obstacle_detection, _tracking, _and_classification)). (Run with `--trace` for a full exception trace.)
The comma is the real culprit in this instance, but I suspect the other strange characters may possess lurking demons as well.