The Only Use of

insidious
in
A Separate Peace

The seniors had been trotted off to the improvised obstacle course in the woods, or to have their blood pressure taken again, or to undergo an insidious exercise in The Cage which consisted in stepping up on a box and down again in rapid rhythm for five minutes.†
p. 36.2

Definition:

not appearing dangerous, but actually very harmful over time

or:

treacherous (dangerous due to trickery or from hidden or unpredictable risks)