From personal experience here is my input on this question!

I am "well rehearsed " in that I practice nearly every day - otherwise I could not play at all ...
HOWEVER even if I have played a piece 1000 times perfectly to the dog in my bedroom I can still stuff it up in front of humans especially if I don't know them too well

Also I can learn words perfectly well & yet forget them from nerves at any time. I do think playing or singing in front of people can be scary & I have only dared do it since September, I seem to be finding it easier though that can come & go.

Nerves and confidence are the key and I can only build those from performing in a supportive atmosphere where perfection is admired but not demanded. I'm grateful to all those who have allowed me my fluffed notes and duff chords , actually my hands still shake with nervousness every time it's "my turn" but I get better at hiding it.