Programming tricks which can be used to hack a website

What are some programming tricks to hack/manipulate the response and make website behave as we would like it to.
One small trick, if we have some non editable fields on any form of website, we can make it editable by accessing the DOM and changing field's attributes.
I would like to more about such programming tricks.

Thanks in advance.
 
Some of the simple exploits that some websites have are :

You can try running javascript code or sql queries in any input box in a website

if the address bar contains some interesting parameters like username ,try changing it to some other username and see what happens

Try giving input which is not expected in the input box
 
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