What is it called when something you previously took to be a mistake turned out to be the correct decision?

Sometimes your “mistake” results in a big success, or you find out that it actually was the correct way of doing it. I sarcastically call this a “correct mistake”. What do you call it? I don’t know if you can call it a “blessing in disguise”.

A common expression is: a "happy accident" (US).

(a few days later...)

My husband just reminded me where I got that saying: Bob Ross- the guy with the mesmerizing painting show... I don't know if he originated it, but he sure made good use of it.

Dumb luck:

good luck that happens by chance, without you planning it at all

or even more incredible, sheer dumb luck:

When something happens by sheer dumb luck, it is considered to have happened unintentionally and without planning.

Also, possibly [a lucky accident]or a fluke:

a stroke of good luck (“Whose run-away horse he had stopped …by the merest fluke,” 1889). ...it seems a small jump from meaning “guess” to “lucky shot" (in billiards), and I’d say that dialect word is almost certainly the source of this kind of “fluke.”

Possibly a reversal of fortune:

an act or instance of reversing; The point at which the action of the plot turns in an unexpected direction for the protagonist.

Serendipity:

the occurrence and development of events by chance in a happy or beneficial way

You can consider these phrases that are directly related to mistakes with good outcomes:

lucky mistake
fortunate error
happy fault

There is also a Latin phrase (used in English), felix culpa, which is usually used in religious contexts. Its literal translation is "happy fault".

An apparent error or disaster with happy consequences.

But there seems to be a felix culpa happening here as well.

[oxforddictionaries]

Felix culpa is a Latin phrase that comes from the words felix (meaning "happy," "lucky," or "blessed") and culpa (meaning "fault" or "fall"), and in the Catholic tradition is most often translated "happy fault," as in the Paschal Vigil Mass Exsultet O felix culpa quae talem et tantum meruit habere redemptorem, "O happy fault that earned for us so great, so glorious a Redeemer."

[Wikipedia]

The luck of the Irish

Because the Irish are famous for landing on their feet, in whatever dire or desperate situation they may find themselves in.