These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Accidental Innuendo: Level 69 of Balloon-o-Rama has pink balloons arranged in the shape of a heart, which soon fills up with white balloons. Ironically, it's also a That One Level for many due to the fact all balloons regenerate after popping them. Three times.
Demonic Spiders: Virtually every enemy could qualify in Dog on a Stick without unlimited lives on, but the beach balls and the hedgehogs are the worst. The beach balls can only move downward, but they make up for it by moving exactly in the direction you never want them to, and the jumping-through trick mentioned below doesn't even work on them. Oh yeah, they also infinitely spawn again once they fall, meaning you can't get rid of them. The hedgehogs are stupidly fast and actually outrun you, and since they don't hop straight up once before going to another space, you never know where they're going. They're also really difficult to get rid of on levels that have switches to trigger the appearance of platforms. While you can jump through them, they're likely going to come right back and get you anyway.
"Putt-Putt, save the zoo, Putt-Putt, save the zoo, Putt-Putt, save the zoo, Putt-Putt, save the zoo..." And it goes on and on and on.
Some of the in-game songs could also qualify, particularly "The Zoo Song," "The Time Continuum," "Go Putt-Putt," "Under the Big, Big Top," and "I've Got a Secret."
Ensemble Darkhorse: Rover and Mr. Firebird, to an extent. Both are well-liked characters, but they didn't receive much attention. Rover is a big side character in Putt-Putt Goes to the Moon, but his only appearances afterwards were working at an ice cream shop in Enters the Race and Pep's Birthday Surprise and attending the party at the end of the latter game. Mr. Firebird wasn't treated much better, as he only shows up in the beginning and end of Moon and Putt-Putt Travels Through Time, in the One-Stop Fun Shopstorytime coloring book storyA Visit to Mr. Firebird, and in the Tie-In NovelPutt-Putt: Lost In Time (he's also mentioned by Putt-Putt in Pep's Birthday Surprise, and his factory/lab is the setting of Balloon-o-Rama's Levels 21-30).
Good Bad Bugs: In Dog on a Stick, if you jump towards an enemy at the same time it jumps towards you, you can go through the enemy. Especially useful in levels with one-square wide paths.
That One Level: Balloon-o-Rama is debatable on this subject, but Levels 61-70 and 101-110 come to mind, the former mostly because of Level 69 mentioned above, and the latter because the levels in that scenery take forever to finish.