Learning that the man who voices Seto Kaiba is also the Japanese dub actor for Castiel on Supernatural.
And discovering that the voice actor for Spike Spiegel, Koichi Yamadera, is the official actor for Donald Duck in Japan
Chiisana Tantei
xxxxMinami Takayama, Kappei Yamaguchi, Kenichi Ogata, Megumi Hayashibara, and Noriko Hidaka have all had roles in both Detective Conan and Ranma 1/2.
Heck, for that matter so have Yamadera, Rei Sakuma, Sakiko Tamagawa, Yumi Touma, Takeshi Kusao, Kikuko Inoue, Shigeru Chiba, Hiromi Tsuru, Takehito Koyasu, Toshihiko Seki, and the late Hirotaka Suzuoki and Kaneto Shizawa.
Hell, considering how long Detective Conan has been on (over 15 years!) and how many different characters/suspects show up per case and storyline, I think everyone and their mother has been on it.