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.