Learn German: Das war wohl ein Fehler. - That was probably a mistake.

In this sentence, wohl acts as a modal particle. It adds a sense of assumption or probability, similar to saying 'probably' or 'I guess' in English. The speaker thinks it was a mistake but might not be completely certain.

Doesn't wohl usually mean 'well'?

It can! When used to describe health or comfort, it means 'well' (as in Ich fühle mich wohl - I feel well). But when dropped into the middle of a sentence as a conversational particle, its meaning shifts entirely to 'probably' or 'likely'.

Why is it ein Fehler instead of einen Fehler?

The verb sein (here in its past form, war) does not take an accusative direct object. Instead, it acts like an equals sign linking two things, so the word that follows stays in the nominative case. Since Fehler is a masculine noun, it remains ein Fehler.

Why use the simple past war here instead of the spoken past (ist gewesen)?

While German speakers usually prefer the spoken past (Perfekt) for everyday conversations, the verbs sein (to be) and haben (to have) are big exceptions. People almost always use their simple past forms (war and hatte) even in the most casual speech.