Olvidé / se me olvidó
I've see both of these constructions before to say that "I forgot" something, can someone explain to me the difference between the first and the second, the so-called "accidental" se pronoun? Is it a matter of emphasis, that I accidentally forgot? And if so, isn't the very fact that I forgot obvious enough that it wasn't on purpose?
8 Answers
It is a matter of perspective. The first construction puts you on the foreground as the subject ([Yo] Lo olvidé), making you the active responsible for not being able to remember, whether it is because you wanted to forget, you did not try hard enough to remember, or because you accidentally forgot it.
In the second one, the subject of the sentence is the memory that vanishing from the brain (Eso se me olvidó), and you are on the background, being the one (the indirect object) affected by such accident. This time one can only assume that it was an accident with you as a "victim".
I just did the lesson on this type of phrasing in Spanish. It exists in English as well, but when we are children, the adults around us discourage it. E.g., "My book got lost!", of course, the book did nothing, I lost the book; so teacher, mommy and daddy all tell us not to say that. Unfortunately, 1) We continue to excuse ourselves this way whenever we can get away with it, and 2) Sometimes the real culprit cannot be identified - "the book really did get lost", not because I was careless, but because it was moved (by person(s) unknown) to the table where books that were being given away were kept, and "it got given away"! 
In Spanish, using the "Se" form kind of lets the "perpetrator" of the "accident" off the hook. Accidents are expressed euphemistically.
Example: Se me olvidó el libro. is literally like saying
" The book forgot itself to me." In other words, it's the book's "fault" not yours.
Other verbs common used this way are: Perder, Romper, and Caer. We have nothing like this in English, so it is very unique (and fun!)
Hi, Thesilent!
This issue was discussed and explained in a previous thread:link
The difference between the two constructions is that of active and passive voice; in terms of usage it's a means of subtly shifting the blame away from yourself (kids caught doing something naughty use it all the time) - consider the difference between
"I lost it" (lo perdí) and
"it got lost" (se me perdió).
¡Eeee olvídalo!
Lo olvidé.
Se me olvidó.
Se me cayó.
Se me perdió.
Lo perdí.
Lo dejó caer.
Se le cayó...
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Bump for other answers hopefully, I asked this at a late hour.