I have a mains socket in my garage, which has a 4-way adaptor plugged in. The adaptor has a light built into it, which is on right up until I plug something into it. In fact, it's on until the thing I plug in is turned on. Then, the item will not work, and the light is off.
The 4-way is one known to work - it does the same thing with multiple 4-ways.
Any ideas?