Ytdyklly

I don't mean damage caused to the socket and prongs, but only to the appliance itself.

Typically an appliance will not be damaged if it is currently ON and then you unplug its power cord. If you plug it back in the device will simply resume operation as if it was ON.

I say typically because there are some cases where this may not be the case. There are many devices such as computers and some other electronics devices that almost always recommend that you interact with the device to gracefully shut it down before you think of yanking the power cord out of the wall. This graceful shutdown process ensures that the device has finished all critical internal operations before losing power. This could include operations such as closing open files or saving important system state to a storage memory.

With many of today's appliances having more and more smart electronics it can be more of a concern that you not arbitrarily pull power cords. A well designed appliance from a reputable manufacturer will have made provisions in the electronics design and software so that the device does not get bricked when the power is suddenly lost. On the other hand as more designs are done by less experienced people this possibility of damaging operation of an appliance by sudden loss of power is more likely than ever.

To make my case let me comment that the ever more popular Raspberry Pi (rPi) microcontroller board is leading many people to think of embedding devices like that into appliances and other gadgets. It is a known fact that removing power from an rPi board can brick the boot media on its micro SD card and keeping it from being able to reboot when power is restored.

It depends on the device.

To take Michael's example of the Raspberry Pi, Flash memory (which works like RAM) is destructive on reads, so to read data you have read it out and then write it back. If you pull the power when it's in use you might be mid-cycle and lose data. Many SSD(flash) drives for more expensive computers have a capacitor that keeps the power on just long enough to complete the cycle if the power goes out.

Appliances don't generally have this issue because they tend to use ROM chips instead. A power failure doesn't matter because the data on the chips isn't volatile. If they are using Flash memory anywhere, it's probably not for mission-critical parts (an appliance is something you're likely to repair and warranty repairs are expensive to companies).

Some electronic devices with on off switches can be damaged when being plugged in , the arcing can damage static sensitive devices. Many electronically controlled devices have debounce circuits because even pressing a switch causes spikes, but if plugged in with the switch on the spikes are many times higher as the electronics power up so at least when plugging things in it will reduce the possibility of damage if the power switch is turned off.

Another example of needing to be careful is in situations where heat removal is involved (such as internally driven forced-air cooling of a video projector bulb). In these cases, to prevent short-term (or longer-term) damage, you need to wait until the device has had a chance to properly cool itself down internally.