Legally, you can only work in a school that you have a valid work permit for. That means, unless you have a minimum of 14 hours, and a valid permit/arc, you are teaching illegally and as such, can be deported. This usually doesn't happen as raids seem to be few and far between, but it's best to keep in mind whenever picking up a few extra hours.
My understanding of how subbing in Taiwan works is that it is officially illegal and could lead to deportation.
However, the chances of getting caught are quite slim because you would need to be at the school when it got raided by the police. The schools tend to know ahead of time about the raids, and they generally only happen if someone has reported the school for hiring illegal teachers.
I have only heard of this happening to one person and he was a long term teacher at that school, not a short term sub.
I hope that answers your question.
