In a previous session, one of my players used Sudden Bolt against a Living Sap. Living Sap's have immunity to critical hits. They rolled low on their Reflex Save, and got 11 less than the spellcaster's DC. I ruled critical failure and gave Sudden Bolt double damage. One of my players had assumed that their immunity to critical hits would make them immune to critical failures (they didn't fight too hard since it was a fairly beneficial ruling :) ). Was I right to rule that way? (I've included my reasoning as an answer on the off chance that I'm right).
Oozes are not immune to critical failures
When you make an attack...
This would also exclude Demoralize or Feint from counting as critical hits, although that's not particularly relevant for oozes. This would also mean that critically succeeding at a Trip would count as a critical hit*.
*as Yopi Lapi pointed out in the comments, the Immunity Rules clarify a bit more what immunity to critical hits means:
Immunity to critical hits works a little differently. When a creature immune to critical hits is critically hit by a Strike or other attack that deals damage, it takes normal damage instead of double damage. This does not make it immune to any other critical success effects of other actions that have the attack trait (such as Grapple and Shove).