Ok so I was using the Psydex to look at prinplup's level up moves as I progress on a new game on my Diamond version. While I was playing I leveled up to level 19, and I thought it was going to be bubblebeam, but it was Bide... I was thinking "how has nobody noticed this error over such a long time" so I checked it on bulbapedia and they have it listed as level 19 as well. I did some more searching, and other sites say that level 19 is Bide which is what I just learned on my prinplup, and level 24 is bubblebeam. I will edit my post when I get to level 24 which wont be long, but I think I am correct on this random error:

Level 19 = Bide not Bubblebeam
Level 24 = Bubblebeam not Bide

If you look carefully under the "DP" column (and NOT the P or HS columns) of Prinplup's level up moves, you'll see level 19 Bide and level 24 Bubblebeam. They have been switched in Platinum, HeartGold and SoulSilver. The Psydex is correct and there is no error here.