I don't know if this counts as mischief, probably not, but it is work-related and made me smile.
As I was stacking firelighters on a shelf, I noticed that the boxes said "Not for children" on them.
Pretty obvious, I thought. Everyone knows that firelighters are
rubbish for setting light to children. Matches work much better!
[N.B. I hope everyone realises that I don't actually go around incinerating the neighbourhood kids - I just thought the linguistic aspect was amusing!]