You can choose up to 3 items in the poll (worst, 2nd worst, and 3rd worst) as you desire. To elaborate on the items themselves:
2-way or 3-way block-type adapters without over-current protection
A 2-way version is common here in Australia, and 3-way versions are prevalent in North America. The problem is that they make it extremely easy to unwittingly overload extension cords (which in Australia are usually rated for 10A, while the circuits supplying the outlets can each deliver 16A or sometimes 20A between them) — and 2.56 to 4 times more heat (as power dissipated in a given resistance — be it a resistor used in electronic devices, the element of an electric heater, the windings of a power transformer, or a simple length of copper cable — is correlated to the square of voltage across/current through it, as I'm sure us technical types already know) is not small under any reasonable analysis. In the UK the mandatory use of fused plugs protects extension cords (although it's not inconceivable that socket contacts can still overheat, given oxidation) from attempts at overloading, while most of the rest of Europe sidesteps the issue by fixing the total circuit current at 16A and requiring all power strips (or power boards as they're known in Oz) and extension cords to safely handle all of that. Ironically (remembering some of those rather comical Public Service Announcements with ludicrous numbers of those adapters plugged into a single power strip), downstream of a power strip is actually the safest place to use them; a standard Australian "power board" with a 10A input cord and circuit breaker will guard against sustained overloading of any downstream extension cords unless they were made with undersized conductors in the first place.
Autotransformers for step-down/step-up applications
As explained here, you have to be careful about substantial transformation ratios, especially in the presence of antique equipment with capacitors that can fail deadly (or a chassis that was live by design from the get-go) — and let's not forget the saturation that will usually result from running a 60Hz transformer on 50Hz.
Compact Fluorescent Lamps for retrofit application in incandescent sockets
I don't see the need to re-iterate on this one, as it has already been discussed in detail. But it is particularly scary that they appeared (in the USA anyway) to be exempt from the normal fire safety requirements…
Bayonet Cap and Edison Screw light fittings
These should have been banned by 1980 to be honest. I hate these "grandfather" provisions that stick around for decades past their time.
Socket covers for BS 1363 (as used in the United Kingdom)
It seems to have escaped the awareness of many companies that the BS 1363 outlets are child-safe by design — and have been since the standard was introduced. Combine that with (quite literally) regulating "plug-in" covers as though they were toys, and this is what you get. They have never been very effective at guarding "flush" sockets (also including the Australian and North American types, among others), anyway (people have long been dangerously prone to underestimating the physical abilities of toddlers).
Universal sockets
Let's just say that there are reasons reputable companies don't make them…
