Pretty sure sad panda is from South Park:

"Say "hi" to Sexual Harassment Panda!" -- Mr. Garrison
"Hi, Sexual Harassment Panda." -- The class
"Did you know that when one little panda pulls on another little panda's underwear, that's sexual harassment? That makes me a sa-a-a-a-ad panda" -- Sexual Harassment Panda
"This is freaking me out, dude." -- Kyle [to Stan]
"And when one little panda puts his furry little willy in another panda's ear, that makes me a very sad panda. Now, I'm going to pass out these booklets, and we're going to go through each and every sexual harassment law." -- Sexual Harassment Panda
"Awwww!" -- The class
Frickin awesome episode.