Recently received my end of a whistle trade, and finally had a few moments to sit down with it earlier this evening. Strange thing happened… I picked the whistle up and straight off I played a fair stretch of, well, a tune that I have never played or set eyes on before. It more or less just came out without any thought at all, and hardly a mistake (- I think). It’s some slow air - I know not the name - and I seemed to know how to play quite a bit of it. Like I said, I don’t know what tune it was, but it did seem vaguely familiar, as if I have heard it before. My friend remarked on how lovely it was, and I just set the whistle back down, a little stunned. Now, several hours later, I don’t even remember how it went. Anyone else ever had an experience like that?
did you forget to clean the ghost out of it? if not, you’ll have that (it happens when someone dies playing a whistle; and frankly, who wouldn’t want to go that way?)
Sure, it happens at sessions all the time. In addition to the tunes you actually have, there’s a “cloud” of tunes you sort-of have, in a passive sense. They’re in your head and in your ear to some degree, but maybe not under your fingers. Someone launches into one of these, and you think “Oh, I know this one”. So you join in, then realize it may be the first time you’ve ever actually played it.
For a seasoned player, this experience merges with the ability to pick up tunes on the fly after a few hearings. In any case, what you’re describing is perfectly normal, and not uncommon. Especially if your whistle is haunted. 
And another variant:
This is one that Mr Theflute can verify. Once, many years ago, myself and a guitarist, being in a heightened state of awareness at the time, started playing. We played a brand new tune, perfectly, first time, with every chord right, and a complete and, though I say so myself, beautiful melody. A friend of ours had just given us the words to The Hundred Men of Haswell, and wanted a tune for it. I’d just read the words, but had forgotten when we started playing that I was supposed to be composing a tune for those words. As it turned out, the tune fitted perfectly and, whilst it has a wide pitch range, also exactly fitted our friend’s vocal range. Fortunately there was a tape running, so we didn’t lose the tune despite our heightened state of awareness.
This reminds me of a quote from singer/songwriter David Wilcox (the US version, not the Canadian) when asked why he prefers luthier Jim Olson’s guitars, “People ask me why I like Olson guitars, and I tell them that they have more songs in them. It’s really true; they do.”
Well… I am certainly more inclined to accept a rational explanation, like the one MTGuru offered, but I have to say that what struck me about the experience was that it seemed exactly like what is quoted above - like the whistle had the tune in it.
Still, I can assure everyone that I’m haunted plenty more by the tunes I can’t play, than I am by the one that spontaneously erupted 
Or the tunes you can play perfectly once, and never again 
John
rational whistle players? …rational musicians of any sort?
In what far corner of the Never-Never do you tuck-in at night???
← Hey, that’s my pony!!
With enough thrust, pigs fly just fine.
I’ve experienced similar. I have about a dozen different whistles and flutes, and with each, it seems there is one or more tunes that “it” likes to play, that seem more naturally suited to that instrument than the others.
I’m still hoping to be run over by Andrea Corr on her motorbike.
Best wishes.
Steve
Before I started playing whistle, I had just bought a really nice NA flute.
The moment I picked it up and blew into into, a beautiful and haunting tune emerged. Still play it whenever I pick up that flute. It’s the kind of tune that stays with you, residing in your heart and memory.
So yeah, it’s definitely possible for an instrument to provide the tune. Isn’t it great that two seperate things can come together to make something so beautiful as music? Or perhaps it’s the spirit of someone long ago, who is guiding my fingers to play a tune they themselves played upon the plains or from a mountain top or forest.
Wherever it comes from, I’m glad it’s there.