The porn movie that Krusty and Chief Wiggum saw (and the former being busted by the latter before "settling" it out of court), Debbie Does Springfield, is most likely named after the real life porn film Debbie Does Dallas.
Johnny (the soldier who got shot while writing a Valentine's Day card to his girlfriend) sounds like Jeremy Freedman (aka the Squeaky Voiced Teen).
Skinner's reaction to Johnny's death resembles the game over phrase from Metal Gear (it's not a true reference, however, as the episode aired prior to the release of Metal Gear Solid in 1998).
Bart's off-color candy hearts read: "Luv Sux," "Prize Pig," "Kiss Off," and "Eat My Shorts".
Despite this episode airing in 1993, it actually takes place a year earlier due to Valentine's Day being on a weekday and Krusty having his 29th anniversary from 1963 (1992). As of February 19, 2019, it has actually been 56 years now since Krusty started his show.
This is the first episode to have a censor bleep meant to censor profanity (in this case, Sideshow Mel whacked out on wowie-sauce).
Lisa erases another kid's name on the card before re-signing and giving it to Ralph.
Lisa's romance with Ralph is referenced several times throughout the series.
According to Al Jean, the "I Choo-Choo-Choose you" card is an actual valentine's day card he got in third grade.
Ralph spills chocolate ice cream on Lisa's dress, and according to Chief Wiggum, nothing can get chocolate out, but later when Krusty is talking with Ralph on the audience, Lisa's dress is clean. Lisa could have found a way to get the chocolate out; after all, Chief Wiggum's judgements are often poor.
When Bart is pinpointing the second Ralph's heart broke in half, Ralph moves 10 times (10 frames) while Lisa moves only 5 times (5 frames). As they were in the same footage, they should have moved the same amount of frames.