How can arcane prison miss?

[Morgueart-kul-tiras (Morgueart)]

it has a 100% hit chance?

[Reyn-undermine]

I don’t know of any pet that has an Arcane Prison ability.

There’s other Arcane abilities with 100% hit chance, but that can be affected by opponent using some sort of block, or some Weather effect like Sandstorm that reduces your accuracy.

[Creeks-frostmourne]

what pet has the ability arcane prison?

[Symbioserous-tanaris (Symbioserous)]

There are quit a few abilities that reduce hit chance below 100%. One of them must have been in play.