Pretty simple, a joke turned into an idea and here is a rough draft:
Perturbing Spell (Metamagic)
Your spells manifest with unusual appearances, distracting those who witness them.
Benefit: Your spell's effects contain a powerful figment, such as a summoned creature dancing wildly as it fights, or a conjured cloud making loud snorting noises. Any creature within 10' spell level that observes the spell's effect must make a Will save, with a DC equal to the original spell (Spell Focus: Illusion applies). Creatures who succeed are unnaffected, but those who fail take a -2 to AC, can not take 10 on skill checks (even if they had an ability that would allow them to), and must make concentration checks at the same DC as the Will save to cast spells or use spell like abilities.
Spells with instantaneous durations create perturbing effects for 1 round. Spells with greater durations create the effect for the full duration of the original spell. Spells with durations of permanent or that create permanent effects instantaneously have perturbing effects for 1 round per caster level. This feat can not be used on spells that do not have some sort of physical presence or visual/sonic effect.
--------
Any fine tuning? I'm still trying to figure out the metamagic cost of this. It's a decent effect, but the range and duration is subjective to the spell, and allows a save to negate. I was thinking +1, but I would gladly hear any constructive commentary.