After failing the mock exam multiple times, when I actually took the real thing, I got a result I hadn't expected.
Literal
Mock-exam [to-に] several-times-[too-も] fail-and, actually take-[try-てみたら], unexpected result [subject-が] came-out.
Two conditional-like forms layered together. 何回も ('many times') uses も with a number/counter to mean 'as many as X,' 'multiple X.' ~てみたら is ~てみる ('try doing') in its ~たら conditional form — 'when [I] tried X,' 'upon trying X.' The nuance is 'here's what happened when I actually tried it.' 思わぬ is an attributive negative form of 思う ('think') using the archaic ~ぬ negative suffix — surviving today in fixed phrases like 思わぬ結果 ('unexpected result'), 思わぬ出来事 ('unforeseen event'). Much more literary than 思わない.