Note cancelation I was playing an Irish whistle with a worship band. I was playing though a vocal mic and listening with in ear monitors. On a high pitch, the note seemed to disappear.
Is this a natural phenomenon or just a possible error playing the whistle?
 A: I think it is very plausible that you would get phase cancellation effects for high notes in a setup like that.  I can't be certain that this is what is happening, but it could easily be.
If I'm correct the high note of an Irish whistle is around D$_7$ which is around $2.3\,\mathrm{kHz}$. This gives a wavelength in air ($\lambda = c/f$) of about $14.5\,\mathrm{cm}$.  Well, the distance between where the sound originates and your ear is going to be of that order, or perhaps rather more (certainly it it no more than a few $\lambda$), and it's going to be fairly constant (you're wearing in-ear monitors which are fixed with respect to your ears, the whistle does not move about much as it's attached to your mouth).  It's not possible to say anything  useful about what happens to the phase of the signal in the microphone-amplifier-headphone chain, but it will very likely be consistent note to note.
So this is a nice region for possible cancellation: the distance between the instrument and your ears is of the same order as the wavelength of the sound, is fairly fixed, and of course the wavelength changes by a good chunk of the distance between notes (the wavelength of C$_7$ is about $16.3\,\mathrm{cm}$ for instance).  So you're quite likely to find yourself in a position where the sound from individual notes from the instrument and the sound from the in-ear monitors is able to cancel (or alternatively to reinforce!).
Of course this will only work if the sound that reaches your ears is at approximately the same amplitude from the instrument itself and the monitors: if the monitors are much louder than the instrument (which may be because you've turned them up, but may also be because they seal fairly well, so not much sound is leaking in from the instrument directly), or much quieter you won't hear this.
And finally there are a lot of weird effects which can go on whereby you hear the instrument through odd pathways: I used to listen to tuning forks in noisy environments by touching the holding-part of them against my teeth for instance, and I'm guessing that some of the sound of the whistle will reach you via your mouth and the inside of your head.  That alters the effective length of the path, but I don't think greatly alters the possibility of cancellation effects.
I think it's possible to test this theory.  Get someone else to wear the in-ear monitors, and to stand rather close to you while you play, and then to move slowly around by a few tens of centimetres.  If there are cancellation effects going on they should find quite noticeable 'dead spots' where the instrument seems much quieter for specific notes.  (This is an experiment you might want to wait until CV19 is over to do...)

(There are some other possibilities, one of which is that the audio system you are using has feedback-detection things which are falsely detecting certain notes from the whistle as feedback and doing some fancy notch-filter thing to remove them.  And also you could just be playing bum notes, of course, as you suggest.)
