Dust Covers have been the most useless hardware add-on ever produced for CPC computers. These accessories have been advertised to "protect" the CPC's dust-resistant sealed membrane keyboard, or even its vacuum-based monitor against dust.
History
Although being completely useless, dust covers have been (in germany, at least) almost more popular 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 déclassé).
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), protective foils for LCD screens (first decade of 21st century), and sliced bread (thirties).
Pictures
CPC 6128 with Dust Cover
Sealed 100% dust-resistant 6128 keyboard membrane
Legal Status
There are no known laws against producing, distributing, or even using dust covers. However, users are likely to lose their precious warranty if a computer catches fire because of covering its ventilation holes after forgetting to 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?