Dust Covers have been the most useless hardware ever produced for CPC computers. These accessories have been advertised to "protect" the CPCs dust-resistant membrane keyboard, or even its monitor against dust.
History
Although being completely useless, dust covers have been (in germany, at least) almost more popular CPC accessories than joysticks. Whereas, it had to be a real dust cover - one needed to buy it (using old newspapers or worn T-shirts would have been declassé).
Today, Dust Covers are more or less unknown. But, they have left some scars: Many people still feel irrationally guilty when exposing computer hardware to dust. The dust cover phenomenon (eighties) can be compared with the hype for equally useless mouse pads (nineties), and protective foils for LCD screens (first decade of 21st century).
Pictures
Legal Status
There are no known laws against producing, distributing, or even using dust covers. However, one is likely to lose any warranties if a computer catches fire because of covering its ventilation holes after forgetting to previously switch off the power-supply.
Fortunately, the dust cover users never realized that dilemma, otherwise they'd have probably died from a heart-attack when trying to find a solultion to the question: risk the warranty? or risk the dust?