How would one parse the sentence 'Woe betide anyone wearing the wrong colour'!
'Betide' is clearly the main verb, meaning 'happen'; as in 'they waited wondering what might betide'.
But clearly 'betide' is not conjugated like other verbs, otherwise it would be 'woe betides'. But in this usage of 'woe betide...' it doesn't even seem to me to be acting like an indicative verb. It is most frequently delivered as an imperative warning, a bit like 'look out!'.
Should the full form be 'Woe will betide anyone....'