Oh dear, I've been bad...
I well know how it is to sing awry: the very first time the schola in Hobart ever sang at Mass, back in the nineties, a nice old lady toddled down to the back of the church to thank us afterwards, but added, "What was that terrible thing you sang at the start, over and over again?" - it was the Introit!
For the feast of St Philip Neri some years back, while still in Perth, Justin and I arranged with Fr Rowe to have a sung Mass (very kindly, he even went to the trouble of preparing an excellent sermon, which he was delighted to confirm with me afterward - I, who have read many books on the Saint - contained all manner of details I had not previously known). Rather foolishly we had decided on singing Mass V, which we'd never sung before: suffice to say we massacred it, and probably the Propers too (not having practised at all), and at the end, when we went to sing the hymn I'd chosen, it turned out that Justin (playing the organ) had the music for an entirely different setting that neither I nor anyone else had ever heard, with the result that what had been intended to be a rousing finale was instead a complete disaster.
So for those who think I've been too critical - I first of all condemn myself for having on more than one occasion completely ruined Missæ cantatæ. This is true.