When I try to log in, I receive an error message “Your username or password is incorrect”. What should I check?

...I'm checking that I haven't entered the wrong username or password.  
Be careful with upper and lower case! If the key  caps lock shift on my keyboard is activated, the username or password entered will almost certainly be wrong - I can click on the eye to the right of the password to check that I haven't made a mistake.
...I'm checking that I'm connected to the right Teaching Assistants Webspace.
If I work at more than one institution, the address at which I log in and my log-in details are different!
In the same way, I check that I'm logging in at the right place if I have children in an institution that uses PRONOTE; if this institution is the same as mine, I have two separate accesses: one as a Teaching assistant and the other as a parent.

...I check the connection mail sent by my institution.
Some institutions ask their users to change their password every year: in this case, my password from last year is no longer valid. My new temporary password was sent to me at the start of the school year by mail (or possibly by e-mail or SMS).