I recently used an action with two words in camel case like actionTestThis which, in Yii2, translates to a url like ‘mycontroller/test-this’.
But if you’ve put in a special rule for this in your url manager, perhaps redirecting this to another controller
'mycontroller/<action:w+>' => 'mycontroller2/<action>',
You will end with that lovely 404 message
What you need to put is
'mycontroller/<action:w+(-w+)*>' => 'mycontroller2/<action>'