Eclipse The deployment descriptor of the module 'xxx.war' cannot be loaded or found

Eclipse The deployment descriptor of the module 'xxx.war' cannot be loaded
or found

There's been a number of similar questions raised on this error on Eclipse:
The deployment descriptor of the module 'xxx.war' cannot be loaded or found.
This is a very generic error, and after scouring the web I have multiple
different causes and solutions. I'm going to attempt to list them all
here, so others won't have to go through the same thing I did.
If anyone experienced other causes and solutions, please list them here
too. =)
(Moderators, please wait for me to put the answer up before judging this
question)