1) He thought something would go wrong, and something did go wrong.
2) He thought something would go wrong, and it did.
Which of the above sentences suggests
a) that he was thinking of a specific 'thing'. He had a clear notion of what would go wrong.
and which suggests
b) that he did not exactly what would go wrong, but thought something would go wrong.
Neither of them suggests one thing or the other. He might have been thinking about something specific or he might have simply suspected there would be some kind of problem.
Both "something" in the second part of 1) and "it" in the second part of 2) refer to something specific having gone wrong, although we don't know what it was.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.