Mordekaiser, the Iron Revenant, has been a part of League of Legends almost from the beginning: The League of Legends Wiki says he was introduced to the action in February 2010. That's more than seven years of live gameplay, which you would think would be enough time to iron out his kinks. But based on this amazing, agonizingly exhaustive (and overall quite polite) list of "All 180+ Mordekaiser bugs ranked by importance and sorted into 17 categories," that's not necessarily the case.
The author, known as NightWind42 on the LoL forums and Naerlyn on Reddit, said that cataloging Mordekaiser bugs is a "hobby," and it's obviously a pastime to which they're very dedicated. Each entry in the list is assigned a "tier" and includes a detailed description of the bug, a breakdown of its consequences, and information on how to reproduce it. Tier 1 bugs "will happen in every single game and will make you lose at best 5 minions, at worst a few kills," for instance, while Tier 11 bugs "don't affect the gameplay anymore, they are just clarity issues."
That may not adequately convey just how deep this thing goes, so as an example here's the second bug listed at Tier 3, which "are very impactful, but happen rarely. Some others are not as bad but happen often, if not always, and some others are what prevent items to be viable or good on Mordekaiser."
"The drake can be replaced by a normal ghost, the same way a normal ghost can be replaced by the ghost of a drake.
Consequences: If you kill the dragon while having a champion ghost, the champion ghost will be replaced by the dragon's. This is normal, and fine. But that happens both ways around. If you ult an enemy, then kill the drake, and then have the enemy die by the remaining part of your ult, you lose the dragon ghost for something that will be a downgrade in the nigh entirety of the cases. And that's not something you can play around. Because if a fight breaks out at drake, then you have to either save your ult and hope that you can win the fight without using it, or use it and hope that the ulted target will die before your taking of the drake (or not die at all). Do you avoid ulting someone while fighting over the drake lest he die during the DoT of the ultimate and after the dragon's death? Do you delay the taking of the drake to have that champion die before? Do you avoid attacking the enemy you've ulted so that he doesn't die after the drake dies? In my opinion, having to choose between these options is not really healthy, at all.
TLDR: Will either randomly make you lose dragon ghosts, or will force you to play very weirdly around your ultimate during dragon fights.
Reproduction information: Go around the dragon. If you're testing the bug alone, lower the dragon's health. Go find an enemy (have him around or just teleport to him), reduce his health and ult him, teleport back to the dragon (in case you haven't figured, I'm not talking about the summoner spell teleport), kill the dragon before the champion dies, and then as the champion dies, you'll lose the ghost of the drake to get the champion's instead. This has a 100% reproduction rate. Its impact is huge, but such a situation doesn't happen often."
There's also a link to a video.
The depth and thoroughness of the report is remarkable by any measure, and for a non-LoL-er like me it probably outshines the fact that Mordekaiser still has so many jagged edges. Unfortunately, the latter point seems to be—understandably—a source of frustration. "The amount of hours I've spent doing this from the very beginning to now isn't to be counted in dozens anymore, and I'll be honest, I'll be kinda bummed out if all of this goes to waste. So unless some fixes start going through, I don't think I will ever do any work on Mordekaiser bugs again," NightWind42 wrote.
"I would also like to remind [readers of] two Riot statements, lest they be forgotten: 15 months ago, as the drake got removed for Elemental drakes, Meddler said that they'd consider giving Mordekaiser the ghosts of the infernal drakes. No news of it since then. And more importantly: 'We have some time set aside to get these two clusters resolved, but that got pushed back a wee bit due to end of year Assassins follow up :(.' We are now six and a half months after this comment, and probably five months after the final aftermaths of the preseason update. I've seen one bug be fixed."
I've reached out to Riot for comment, and hopefully some information about if and when the Great Big List of Mordekaiser Bugs (Ranked and Sorted!), and will update if I receive a reply.