The target takes a –20 penalty on its next attack roll or check that requires a d20 roll. The action must be one deliberately taken by the target on its turn. Creatures with more than 10 HD are unaffected by this spell.
Check: A check is a d20 roll which may or may not be modified by another value. The most common types are attack rolls, ability checks, skill checks, and saving throws.
Saving throws are also affected.
It does say the action must be one taken by the target on its turn though.
If I cast a spell requiring a saving throw bungle will not apply to the victim because it is not their turn. If the person has to make a saving throw on their turn due to a spell like Hold Person that might require them to make a saving throw on their turn due them failing a previous save then Bungle applies.
Our Wizard always uses it on turns our witch is readying a rough spell or hex so that they tend to fail miserably. Goes like this.
Witch: I ready to cast slumber Hex on the ogre when it tries to attack.
Wizard: I ready Bungle on the ogre when the witch tries to hex it.
Ogre goes to attacks gets hit with Bungle then gets hit with slumber hex at a massive penalty.
