Bob faces a Mythos entity and has to make a Stability test or lose 7 Stability. It's been a long day and Bob is already at -2 Stability. Bob's player chooses to spend 4 Stability points in order to (probably) automatically succeed on the test, bringing him to -6. This means that despite the success, Bob's mind is blasted.
But was this a mythos shock?
Here's basically all the rules say on mythos shocks, and why it matters:
Each time you are blasted by a Mythos encounter or attack (when your Stability drops to between -6 and -11), your Sanity rating drops by 2 points.