One of the sources of "sad humor" is that defensive obviousness that product manufacturers practice in an effort to prevent customers from getting angry because something they assumed about the product was incorrect. Or, it may be that in our litigious culture, manufacturers want to avoid being sued over some issue that any reasonable person would find laughable (hence the source of humor) but that someone out there somewhere could conceivably claim to have assumed about the product.
Example: An advertisement for a toolbox for $14.95 shows it in a picture filled with tools. The caption says, "Tools sold separately."
Example: A set of jack stands includes the instruction, "Raise vehicle to desired height using
an appropriate jack (sold separately)."
Example: A new 5-gallon propane bottle is sold for $28. On the label are the words: "Empty. Does not contain propane." Anyone lifting it or just reasoning from the price should know that, but someone might assume. . . .
Example: A small clock radio includes a sack of desiccant to keep the item dry during shipping and storage before sale. On the pack is a message: "Warning: Do Not Eat." Is that because otherwise, someone would think they were buying a clock radio and a snack?
Example: A razor comes with a protective plastic cap covering the blades. On the package is the note: "Remove protective cap before use." (Hmm. No wonder I wasn't getting a very close shave.)
Another, somewhat similar category are messages accompanying dynamic demonstrations of a product (often a car or truck) warning the viewer: "Do not attempt. Warranty void if product abused in this way." Our hopes are as deflated as the tires after that jump through the air when we read such a disclaimer. Here we were hoping to buy that car just so we could run it off lift ramps at high speed and fly over drainage channels and fallen causeways.
Disclaimer: This blog entry is not guaranteed to make you wiser or smarter or to help you get a high score on the GRE. No warranty is expressed or implied. Use at your own risk. Your mileage may vary. Knowledge sold separately. Wisdom not included.