Maybe it was because my SPD cleats were worn, but with maximum tension on a pedal, I could not unclip - full stop!
I actually had to 'scoot' my bike up the hill I semi-dismounted on because I could not get my foot off the pedal. I must have been a bit thick that day because it did not occur to me to take my foot out of the shoe ...
I eventually caught up with the other riders and had to get
@Ajay to twist my foot round by hand!