When saying "se me olvidó la cartera", we are describing the fact that we forgot the wallet as something that just happened to us, not as something we did. It kind of makes us feel less guilty or less clumsy, and it also expresses we didn't do it on purpose.
- Se me rompió el vaso. (It was an accident that happened to me.)
- Rompí el vaso. (I take the blame for it and I would rather not say it.)
- Ay, se me cayó el agua en el teclado. (I don't want to be blamed for that.)
- Ay, derramé el agua sobre el teclado. (I take responsibility for that: I was clumsy and I'm guilty for having let it happen.)
As for "le enfadarás al maestro", the answer is more complicated:
First of all, for some of us, this sentence is incorrect as a case of "leísmo": the use of an indirect object pronoun with a direct object, but it's the normal use in some regions, mostly in Spain.
- ¿Has visto a Juan? Le estoy buscando. -> Many of us would rather say: "
lo estoy buscando."
- Supe que a Roberto le van a nombrar jefe. -> "
lo van a nombrar"
Now, putting together the indirect object pronoun and the explicit indirect object in the same sentence is commonly heard in many regions and it depends on the situation and the preferences of the speaker.
- Le presté un libro a María. = Presté un libro a María.
- Les abrimos la puerta a los señores. = Abrimos la puerta a los señores.
Finally, although there are regions where "lo enfadarás al maestro" can be heard, for other speakers, the redundant direct object pronoun together with the explicit direct object is not correct. Your proposal "enfadarás al maestro" is the best sentence.
