Pulling the power causes everything to stop in flight, with no warning. kill -9 has the same effect on a single process, forcefully terminating it with a SIGKILL.
If a process is killed by kernel or power outage, it doesn't do any clean-up. That means you could have half-written files, inconsistent states, or lost caches. You usually don't have to worry about any of this because of journaling, exit status and battery backup.
Temporary files in /tmp will be automatically gone if they are in tmpfs, but you may still have application-specific lock files laying around to remove, like the lock and .parentlock for firefox.
Most software is smart enough to retry a transaction if it doesn't record a successful exit status. A good example of this is a typical mail system. If a message is being delivered, but gets cut off in the middle, the sender will retry later until it gets a success.
Your filesystem is probably journaled. If you are moving or writing a file and it dies mid-stream, the journaled file system will still reference the original. The journaled filesystem will make changes non-destructively, leaving the old copy, then only reference the new copy as a last step before reclaiming space the old copies occupied on disk.
Now if you have a RAID array, it has all kinds of memory buffers to increase performance and provide reliability in a power failure. Most likely your filesystem will not know about the caches in the device and their state, so it thinks a change has been committed to disk, but it is still in the RAID cache somewhere. So what happens when the power dies? Hopefully you have a functional battery in your RAID enclosure and you monitor it. Otherwise you have a corrupt file system to fsck.
Yes, a few bits can become corrupted in a binary, but I would not worry about that much on modern hardware. If you are really paranoid, you can monitor the health of your disks and RAID with the appropriate tools, but you should be doing that anyway. Do regular backups and get an Uninterruptible Power Supply.
The concept is called n+1 redundancy. The reason three PSUs are commonly used is that it is more efficient to have three 380 W PSUs operating at 50% load and 90% efficiency than have two 760W PSUs at 37,5% load and less than 85% efficiency.
All that said you should consider that the PSU rating is for rough orientation only. For one thing, your system power consumption will typically be way below the peak rating. For the other, the value is usually just a sum of all available power bus ratings (there are plenty in a PC) and thus not useful as such for any kind of precise calculation.
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With multi-phase AC systems (motors, etc.), you're right, bad things can and will happen if one of the phases drops out. However, with computer PSUs, each of them operates completely separately, converting its AC input voltage to a variety of DC voltages for the computer system.
You can safely run redundant PSUs on different circuits, different phases, etc. Doing so is actually a really great idea to reduce the number of components that are fate-shared.