Another 'sweet' ;) kata from Mr Sorbi. Be careful, you're in danger of becoming my go-to kataist.

Very good kata. Nice introduction to this level for me. Simple instructions, but still some hard work. Will look out for your others.

This looks like a case of the instructions not being followed to the letter.

'If one or more of the opponents pieces are sandwiched by the piece just played and another of the current player's colour...'

Emphasis on 'just played'.

In the play sequence that both MariusAFT and jkranking refer to, neither of the adjacent pieces are 'just played'.

Instead, white has cleverly inserted his piece between two of his opponents previously played pieces, rendering his piece safe.

It caught me out too. If I'm right about the reason then this specific error message might need revising as it's misleading.
Good kata, however.

Am I misreading your explanation...or in your examples have you mistakenly marked the fourth group of characters — 01010110 — as an error when actually it is not?

Passed all tests, but CW kept timing out when I tried to submit. Took me longer to refactor my code than it did to write it in the first place. Shaved off a few seconds here and there, cursing you at every turn...until I managed to sneak it under the wire and realised that it was a good lesson in code efficiency. So thanks for that...and apologies for the bad language albeit unheard.