When Elmer drinks the potion, turns various colors and goes spinning around the lab, Bugs says, "I think Spencer Tracy did it much better. Don't you, folks?" This is a reference to Tracy's role as the title character(s) in the 1941 release of "Doctor Jekyll and Mr. Hyde."
The second of only two Bugs Bunny cartoons directed by Frank Tashlin; the other being The Unruly Hare (1945). Frank Tashlin is uncredited because he had left the studio before the short debuted, and Warner Bros. had a rule at the time that former employees couldn't be credited. This is also why Robert Clampett isn't credited in some of his last shorts for WB. Robert McKimson inherited Tashlin's unit after his departure.
There is an obvious edit at 2:59 after Elmer says "Yay! I twapped him!", as the music suddenly skips in the next shot and it looked like Bugs was going to reply before the next cut. It is unknown what was removed and why, so the scene is considered lost.
Bugs says Elmer is "trying to slip me a Mickey." In slang, a Mickey Finn is a drink laced with a psychoactive drug or incapacitating agent (especially chloral hydrate) given to someone without their knowledge, with the intent to incapacitate them. It was named after the manager and bartender of the Lone Star Saloon and Palm Garden Restaurant, which operated in Chicago from 1896 to 1903 in the Chicago Loop neighborhood. In December 1903, several Chicago newspapers documented that Michael "Mickey" Finn was accused of using knockout drops to rob some of his customers.
The signs Bugs holds up are all slang references to craziness: screwball, crackpot, drip, and batty in the belfry. The screwball drawing would be seen again in Duck Amuck (1953).