I agree that pop music is not a good place to look for compliance with grammar rules. I also have seen those particular lyrics in several places and they don't always start with "I'm." It sounds like that in some recordings, but not others. I think it was written just to start "I know I'm no angel..."

Probably "I know I'm no angel", which is good grammatical English and means "I know I'm not an angel" I cannot explain why it starts with "I'm" - it is not correct and serves no poetic purpose. Perhaps his English is very weak. Pop songs can be sources of horrible English. Even advanced poems break rules on purpose. THis example is really just bad English

Do you know the song named "Hello"? There is a sentence with the wrong grammar rule: it don't matter. It should be "It doesn't matter". But Aldele used the word "don't" instead of "doesn't" because "don't" fits the rythem. (: so, maybe this is what you are looking for. Actually, I watched a video on Facebook about the conversation between 2 guys. One said " It's going down". The other said "No, It don't, it don't go down". I felt weird. English is crazy. English writing and English speaking are much different. I think so.