Matrix multiplication randomised verification - error probability
If that's the whole algorithm, only one iteration, then the only bound on the probability of error you can get is that it is smaller than $1-1/n^2$.

Hint 1) Notice that the algorithm only can fail when it enters in the first case you describe, why?

Hint 2) What is the worst possible case for A, B and C so you get the worse probability of error?