When you cast mental jolt and the target fails their save it silently adds a desensitize affect. If you subsequently "big jolt" them again, you get a "they're already desensitized" echo even though the effect is (in theory) supposed to be hidden:

1st jolt:
TARGET staggers backwards, looking completely baffled!
Your mental jolt MANGLES TARGET!

2nd jolt:
TARGET is already desensitized to pain.
TARGET staggers backwards, looking completely baffled!
Your mental jolt MANGLES TARGET!