Tracking down an invalid render array key warning in a Drupal 8 theme

ok the way to see what template is actually causing it, easily, i’m sorry to say i found the cause before i found the easy way to find it. The easy way should have been in the error message itself, but in my syslog output, which was exceedingly verbose, the path to the template causing the problem was truncated!

helpful: https://drupal.stackexchange.com/questions/253594/user-error-attributes-is-an-invalid-render-array-key-when-rendering-image

i just looked for things that cauld be doing what it says, rendering an {{ item }} without it being {{ item.content }} or {{ item.attributes }}. There was nothing so obvious but there was an if items[0]|render — and that was it.