Sinclair Calculator Watch:
Or perhaps the Sinclair Black Watch Prototype, in White, as worn by Clive Sinclair himself, which just went for £800 on ebay:
As you would expect from a Sinclair product, Features of this glorious timepiece are as follows:
* The chip could be ruined by static from your nylon shirt, nylon carpets or air-conditioned office. This problem also affected the production facility, leading to a large number of failures before the watches even left the factory. The result was that the display would freeze on one very bright digit, causing the batteries to overload (and occasionally explode). This fragility has meant that very few working Black Watches survive today.
* The accuracy of the quartz timing crystal was highly temperature-sensitive: the watch ran at different speeds in winter and summer.
* The batteries had a life of just ten days; this meant that customers often received a Black Watch with dead batteries inside. The design of the circuitry and case made them very difficult to replace.
* The control panels frequently malfunctioned, making it impossible to turn the display on or, alternatively, impossible to turn it off - which again led to exploding batteries.
* The kit was almost impossible for hobbyists to construct (and barely any easier for Sinclair's hard-pressed workforce). Practical Wireless advised readers to use two wooden clothes pegs, two drawing pins and a piece of insulated wire to work the batteries into position. You then had to spend another four days adjusting the trimmer to ensure that the watch was running at the right speed.
* The casing was very hard to keep in one piece. It was made from a plastic which turned out to be unglueable, so the parts were designed to clip together. The clips didn't work either and the problem was turned over to a subcontractor. Sinclair later (much later) received a small box on which was written, "We've solved the problem of the Black Watch!". Inside was a Black Watch with a half-inch bolt driven though it.