While confident in the scorpion, I experimented with numbing poison with many attempts on both a willing PC and various NPCs with no visible affects or echoes showing an affect. The PC said that it never did anything at all to them. It also never showed any echoes for NPCs when striking them or on the ticks following a strike.

A sample numbing sting would look like this:

Your stinger lashes forward, piercing deep into PLAYER's skin.
Your numbing sting grazes PLAYER.
PLAYER has some small but disgusting cuts.

Maybe the PC made his save each time, but I doubt that all the NPCs (including much lower level ones) should have made their saves every time.

The saving throw was backwards...