When there's a huge difference between the time spent beating a boss as intended and spamming the main character's post-hit invincibility (as can happen in Mania's very first boss), that's bad game design, no matter how you see it. EDIT: Reworded.
Just found a physics-related oddity that I'm not sure would happen in 3&K (but maybe?): http://youtu.be/TTguAwgm-5s?t=332 (at 5:32)
Yup, that's been there since the initial release. Here's a better look at it: [youtube]http://www.youtube.com/watch?v=UE-3_o7XFrg[/youtube]
Just a cute little oddity for if you somehow manage to get a character to do their win pose when miniaturised in Metallic Madness Zone. http://www.youtube.com/watch?v=8aPCiZmrxeY
I wouldn't really say that we're "whining" because a lot of the issues being discussed here are actually odd and if not, it's simply "other stuff". There are several buggy things that I'd like to point out.. but I don't need to since others are already addressing them all. In all honesty, I'm a "ring keeper" which means I like to collect a massive amount of rings though the stages and that's how I roll. The oddest thing about Mania is that it's super easy to lose rings, particularly from the bosses being so difficult. The PGZ Act 2 boss is rough and hard to keep up with, just one example. I could play through most of Sonic 1, 2, CD and 3&K and not lose all of my rings even during bosses. That's true satisfaction! Collecting rings in the stages now became less important in Mania for me at least because I know they're going to be lost at some point during a boss.