CLAMP Works CLAMP: Series Connections

One thing that attracts me to CLAMP works is the connection/ relation, or the little things that reoccur in different titles.
To clarify, things like Chitose, her dormatory, Clow reed, references to CCS and other series in XXXholic, Mokona, TRC, etc...!

I'd like to dedicate this thread to list down anything of interest, whether it be an item, character, place, event, that were either mentioned and/or appeared in 2 different titles!