Matching power leads
Many many years ago I worked at a large consultancy. Some mobile staff had IBM ThinkPad 240s (the REALLY small ones). And we also offered them USB CD drives as often these were needed "in the field". Great.
Except that they both used the same power plug. And the CD drive was 5V, but the laptop was 12V (or something else higher than 5V).
At least it was that way round as better fried CD drives than fried laptops!