Deep bite from a feeder mouse!:-/

[treeboa]

Because scared and panicked animals are dangerous animals.
You place an unstunned/properly fed and watered/calm feeder into an enclosure, all they're going to do is explore or groom themselves.

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This is, also, a good recommendation to the OP. I have never seen a mouse get immediately aggressive. How long did you leave the mouse unattended in the enclosure? Mice don't usually bite until they've been left so long they get hungry or bored. There's nothing wrong with live feeding when you're careful.