as you said, it depends on the context. At first, one would use "used to" but it is also suitable the use of "would", which is primarily used as "futuro do pretérito" (I would call you every day as a child = eu ligava para você todos os dias quando eu era criança. now, the usage is different).
Eu não pensei que você errase: a person is your idol, but then he tells you some of his mistakes when writing a song, singing, etc. You can use this sentence. (General mistakes).
Eu não pensei que você erraria: it is specific and refers to a singular mistake (eu não pensei que você erraria este exercício na prova).
I will tell you, it is hard to understand and translate these subjunctives both in Portuguese and Spanish , even if you are a native, but emeyr has caught the spirit. We are studying past subjunctive in this lesson, but the meaning is not like that every time. Although it should be.
I disagree a little because you would only say to someone "I didn't think you would make a mistake' after they have just made a mistake and therefore proving your theory wrong. Or do you mean that the unrealized event is the belief of making no errors? Sorry I'm really struggling with the subjunctive!
This sentence, in other words, mean:
- I didn't think you were a person that made mistakes
- I though you were the kind of person who would never make a mistake
So, the version with "a mistake" doesn't sound very good in the first one (the one that follows the same structure as the translation), while the version with "mistakes" sounds perfect.
In the first sentence where "errasse" occurred, the system marked my "erred" as wrong and said I should write "failed". The second time "errasse" came up, in an identical construction, I dutifully wrote "failed", which the system marked as wrong and said instead that I should have written "erred".
In many cases a good fit for the meaning is "err": http://www.collinsdictionary.com/dictionary/english/err
Unfortunately, except in some set expressions, "err" sounds quite formal, some would say archaic, and has mostly fallen out of use. It survives as a useful space saving device in newspaper headlines, though, as a search for "erred" in Google News will demonstrate.