Why Ruber's Villain Song looks more like a mediocre poem. The guy is completely and utterly insane, so of course he wouldn't be able to construct a proper song.
Devon and Cornwall get a tiny example — maybe — following their weird duet about wanting to be separate. Garret reluctantly allows them to come along on the condition that there be "No singing!" Devon inquires how Garret feels "about interpretive dance." The man is blind. The question mark for the Fridge Brilliance lies in whether or not Devon was aware of Garret's disability at the time.
The two-headed dragon: One head has an American accent, and the other is British. Yeah, yeah, it's a kids' movie, but still.
Also, the sword in the stone's magic heals all injuries in Camelot, and even separates the two-headed dragon, but it doesn't restore Garret's eyesight. This is almost certainly a case of Reed Richards Is Useless, however, as the film would have been presenting most Unfortunate Implications to children about handicapped people's worthiness if it had.
Perhaps it only fixed magic-related injuries (restores the weapon-warriors, heals Arthur who was hurt by the Griffon, separated the dragons, and so on). Garret's blindness was the result of a blow to the head during a stable fire.
I wouldn't exactly call King Arthur's injury from the griffon being magic-related.
This is perhaps the biggest fault that the film has, what with the random magical plants, the modern references, and Bladebeak's personality changes... besides the random singing (but that's the executives' fault).
Why didn't Garret use some magic plant to cure his blindness? I think he tried, but the Powers That Be decided it was cool for him to be blind...
It's fairly simple really. The magic at the end doesn't actually heal anyone, it just makes them fit their self-image. Yes, the dragon gets split into two, but they almost immediately merge back together. The mooks still thought of themselves as people, Bladebeak still thought of himself as a chicken and King Arthur thought of himself as having a working arm. Garret, on the other hand, sees himself as blind , so blind he stays.
That actually does make sense. Well reasoned.
How about the healing leaf that Kayley applies to Garret's wound, which magically heals him without a scar? I could actually buy that, more or less, but how does it repair his clothing too?
It was probably simpler to not have to continuously keep track of a tear in Garret's clothes. Fixing the clothes just made it easier on the animators. Plus, its magic. Don't try to explain it.