Here's another trivial question I'm interested in trying to answer: what was the last Warner Brothers cartoon to feature a contemporary, or at least recent, song?
First some background: Looney Tunes and Merrie Melodies began, in part, as an excuse to plug songs from WB movies and WB-owned publishing companies. Even after the cartoons stopped being built around songs, the WB-owned song catalogue was still very important to the scores of the cartoons. Composer Carl Stalling said that when he moved from Disney to WB, the biggest advantage of working at WB was that he could use popular songs, whereas at Disney he could only quote public-domain music. Stalling built up a huge catalogue of song quotes which he would use at appropriate moments -- "The Lady in Red" for any lady in red; "Powerhouse" for factory scenes; "A Cup of Coffee, a Sandwich and You" for any scene involving food -- but he would also work in songs that had just been published or recorded at the time he was writing the scores. So "Little Red Riding Rabbit," a cartoon released in 1944, uses "Lady in Red," "Cup of Coffee," "Powerhouse" and other Stalling staples, but it also uses at least two songs from a WB musical called Thank Your Lucky Stars, which was released just before the cartoon went into production. The songs, by Arthur Schwartz and Frank Loesser, are "I'm Ridin' For a Fall" (heard when Bugs first meets the Wolf) and "They're Either Too Young or Too Old" (which accompanies a chase scene with the famous "Friz Freleng Door Gag"). Unlike most of the songs Stalling quotes, the subjects or titles of these contemporary songs don't have any direct relationship to the onscreen action; they're just there to give the score a contemporary, up-to-date feel. And it works; part of what gave the Stalling scores their special kick was the sense that they were in tune (no pun intended) with the musical style of the time, more than any cartoon scores since those of the early '30s Fleischer cartoons.
But eventually, while the cartoon scores remained great, they sounded less contemporary. There were various reasons for this: Stalling was getting older; WB was making fewer musicals and selling off its music-publishing holdings; there was a longer time lag between the production and release of the cartoons; musical styles were changing in directions that weren't compatible with the Stalling cartoon-music style. (When Friz Freleng wanted a distinctively late '50s musical sound for "Three Little Bops," he turned to an outside musician, Shorty Rogers, to do a score in a decidedly non-Stalling style.) So the WB cartoon music after 1951 or so doesn't use a lot of contemporary music, and doesn't have a sound that anchors it in its time the way the sound of the "Coal Black and de Sebben Dwarves" score screams early '40s, or the way Bugs' rendition of "It's Magic" (in 1951's "Rabbit Every Monday") tells us we're in the era of Doris Day. By 1962, which was the last year of cartoons scored by Stalling's lieutenant Milt Franklyn, the Stalling song library was starting to sound a bit anachronistic: it's the '60s and the cartoon scores are still quoting Raymond Scott and Billy Rose. This might be one of the reasons why, after Franklyn's death, the directors brought in Bill Lava, a veteran composer who tried to give the cartoon scores a sound more in synch with the film music of the era. It was a bad choice, but to be fair to Lava and the people who hired him, the importance of Stalling and his style to those cartoons wasn't really fully understood until some years later. At the time, that style was looked down on as obvious and unoriginal (because of all the song-quoting). Chuck Jones, talking about Stalling in the '70s, said that Stalling quoted songs because that was "the easiest way" for him to come up with a score in a week; it wasn't until later that cartoon directors -- including Jones -- came to realize that clever song-quoting is part of the fun of cartoon music.
Anyway, to answer my question, I believe the last WB cartoon to quote a recent song was "Rabbitson Crusoe," released in 1956 but probably begun in 1954; in it, Bugs Bunny sings "Secret Love," from the 1953 musical Calamity Jane.