Recently reading a comment on a sitecheck thread at the Telenet Breedband Forums a users metioned Mystery Meat. This struck me as an odd webdesignterm and I had trouble putting it in the right context.
Luckely Google exists! Apparently the term was introduced by Vincent Flanders, creator of WPTS as Mystery Meat Navigation (MMN). Navigation that uses Javascript rollovers; a user must rollover the image in order to know where the link will take them. "MMN is confusing. Confusion is bad." (Vincent Flanders).
On his page about MMN Vincent Flanders explains clearly why MMN is not acceptible. He also give a nice example of why MMN is very bad.