I'm doing random CPU battles, and I randomly select stages. Interestingly enough, the game never froze on me when my friend came over to try out Brawl- for the first time, but then again we never got through all of the stages I turned on for random select.
Oh yeah, I forgot to mention, Battlefield sometimes loads with no music at all; Brawl- somehow deleted one of the songs (I've been noticing a lack of Battlefield v2 playing) and I know for sure because My Music freezes the game when I select Battlefield even when there's no SD card in the slot.
There's also a few quirks with Pichu and Ike; Pichu's side taunt sometimes literally has Pichu say "Pika Pika!" and Ike's No Sympathy mode just never kicks in. I also found that Pichu and Roy are both under the name Crazy Hand in Home Run Contest because they didn't Minus the portraits there, but that isn't important (besides the fact that the game again froze when I hung around at the character select screen too long).
I have no idea what's causing these errors and game freezes. It's incredibly annoying that I especially have to deal with the game occasionally freezing when I'm not doing anything wrong. It's a good thing that there's nothing else wrong with the disk... at least I hope so.