PS/2 to USB converters are complex things with interesting faults
My favorite keyboard and mice are PS/2 ones, and of course fewer and fewer PCs come with PS/2 ports (especially two of them). The obvious solution is PS/2 to USB converters, so I recently got one at work; half as an experiment, half as stockpiling against future needs. Unfortunately it turned out to have a flaw, but it's an interesting flaw.
The flaw was that if I held down CapsLock (which I remap to Control) and then hit some letter keys, the converter injected a nonexistent CapsLock key-up event into the event stream. The effect was that I got a sequence like '^Cccc'. This didn't happen with the real Control keys on my keyboard, only with CapsLock, and it doesn't happen with CapsLock when the keyboard is directly connected to my machine as a PS/2 keyboard. Unfortunately this is behavior that I reflexively count on working, so this PS/2 to USB converter is unsuitable for me.
(Someone else tested the same brand of converter on another PS/2 keyboard and saw the same thing, so this is not specific to my particular make of keyboards. For the curious, this converter was a ByteCC BT-2000.)
What this really says to me is two things. The first is that PS/2 to USB converters are actually complex items, no matter how small and innocuous they seem. Going from PS/2 to USB requires protocol conversion and when you do protocol conversion you can have bugs and issues. Clearly PS/2 to USB converters are not generic items; I'm probably going to have to search for one that not just 'works' according to most reports but that actually behaves correctly, and such a thing may not be easy to find.
(I suspect that such converters are actually little CPUs with firmware, rather than completely fixed ASICs. Little CPUs are everywhere these days.)
The second is the depressing idea that there are probably PS/2 keyboards out there that actively require this handling of CapsLock. Since it doesn't happen with the Control keys, it's not a generic bug with handling held modifier keys; instead it's specific behavior for CapsLock. People generally don't put in special oddball behavior for something unless they think they need to, and usually they've got reasons to believe this.
(For obvious reasons, if you have a PS/2 to USB converter that works and doesn't do this, I'd love to hear about it. I suspect that the ByteCC will not be the only one that behaves this way.)