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Or at least it seems to be the cleat.
Today's MTB ride was largely one footed.
A few km in and I suddenly lost the ability to unclip on the right side. Nothing happened up to this point. Friend had no bother trying the pedal so not that. Cleat not moved, still tight. No damage to shoes. Could clip in fine and unclip moving heal inwards.
Cleaned everything up when I got in and visibly everything is fine.
Is it possible that this is just failure through wear of the cleat (or minor damage?). Normally cleats tend to fail by getting sloppy and unclipping? Once had this after a crash where I think I may have deformed the cleat?
Going to order some new cleats and hope that fixes things.
Photo of your shoes and cleats would help
And maybe some idea of the system in use...
 They are m540 pedals with sh51 cleats.
One of the best riding stories from this year was from a friend who had visited the US for holiday. He had a great things to say about the full enchilada trial. His first mechanical was a cleat problem so that he couldn't unclip a pedal. Second was a slow puncture. He had to stop periodically so his girlfriend could pump up his tyre 😂