Seen in a manual for a consumer electronic device:
Do not connect incompatible products.
Wow, it never would have occurred to me that incompatible products might be… well … uh… incompatible!
I’ve always been amused by the airbag warnings on car visors:
WARNING: Death or serious injury can occur!
This is generally followed by a direction to read the additional details on the other side of the v-isor. But I think juat that single sentence should suffice.