'Toteworthy' nailed it...the problem with the jacket wasn't the mysterious unknown alterations, it was that I put the collar on upside down!!
Spent a bit of time spent unpicking and resewing and...TA DA...the humongeos error has evaporated:
Obviously, the guide sheet is a necessary friend for this project. And, since I recollected that page one/two of the guide sheet was missing from the envelope when I originally cut the jacket out, I signed up for Club BMV and ordered another copy of the pattern.
I'm going to have to make this again, correct from the beginning, and see how it turns out. ;-)
Thanks to all my internet buddies who offered consolation and advice, and especially to 'Toteworthy' for spotting the actual error!!