"In the case of shutdown using the button, no hardware damage is possible (noting that corrupted storage content is not hardware damage).
In the case of power failure, damage is not caused by the sudden loss of power. But it absolutely can be caused by the fluctuations of voltage and current, both up (at extreme levels these are called spikes) and down (brown-outs) that inevitably occur on the line immediately prior to power loss as the power company's equipment fails. These fluctuations can be observed often in your house lighting as it flickers before losing power.
In addition, since voltage fluctuation can sometimes occur during normal operation, without total power loss, a high quality PSU or some kind of power conditioning UPS will help preserve the longevity of your equipment.
Dirty power, or EMI noise on the power line, is also damaging to sensitive electronic equipment - watch where you plug in that treadmill or other large motor device.
Lastly, power fluctuations are more prevalent on low voltage (110 - 120 V) mains than on high voltage, such as 220 - 240 V systems."
"With modern systems, no - as long as you're using the power button, its designed to have a graceful shut down. There's some possibility of data corruption, since your system would not have the time to save everything it was working on.
On the other hand, the very reason you're forcing a shutdown might indicate hardware or software problems already existing.
EDIT: thats practically a seperate question. Thinkpads arn't cheap for a reason - they're tanks, and have great warranty service. Pulling the cable, and closing the computer, is... well stuff they're designed to do. If you switch off the mains, and turn it back on, the Power brick will handle it, gracefully. Unplugging is within design specifications, hell, i suspect lenovo might have a machine to test just that. I'd say none of that sounds dangerous, outside the risk of data loss."
"The accepted answer covers electrical issues, but not mechanical ones. I thought of editing the answer, but I'll add it here for now.
Hard Disk Drives
On old computers, definitely yes. On new ones, probably not. Wikipedia covers this really well:
Modern HDDs prevent power interruptions or other malfunctions from landing its heads in the data zone by either physically moving (parking) the heads to a special landing zone on the platters that is not used for data storage, or by physically locking the heads in a suspended (unloaded) position raised off the platters. Some early PC HDDs did not park the heads automatically when power was prematurely disconnected and the heads would land on data.
Optical Drives
This probably depends on the type. I couldn't find anything authoritative, so this is just my experience.
On the slimline drives, unlikely since the disc is physically attached by the user. On the standard tray drives, not that I've experienced. On the slot-load drives, definitely yes.
Tapes Drives
Yes. Installation guides for tape drives often mention a UPS (i.e. battery backup) as a recommendation.
To give some perspective, an LTO-3 drive feeds 680 m (2,231 ft) of tape in 80 seconds."
"Lets just say that hardware damage is possible but unlikely.
And you have to separate the two cases
When the power gets cut off suddenly for example when a fuse blows there is a small chance that a power spike damages the computer. when holding the power button for five seconds you could in theory give the computer time to prepare (don't know what really happens)
That said, a corrupt file system due to a sudden shutdown is actually software issue, it's not the physical disk that is damaged."
Physically? Not very likely. But filesystem structures could be left in an inconsistent state, leading to operating system or data errors when the system is used later.
