[bluGill]

The problem is QA tends to get to scripted. You need to exercise each corner of the product, thus you test A, B, C in that order - so you never find cases where testing C, A, B breaks, or any other permutation. (to be fair with any complexity it is impossible to test all permutations)

[adrianratnapala]

That's one part of it.

Another part is hiring a breed of test engineers who like breaking stuff and have a knack for it.

[ebiester]

We call it exploratory testing.