ExpressionChangedAfterItHasBeenCheckedError Explained

Question

Please explain to me why I keep getting this error: ExpressionChangedAfterItHasBeenCheckedError: Expression has changed after it was checked.

Obviously,

Answer 1

I got this error because i was using a variable in component.html which was not declared in component.ts. Once I removed the part in HTML, this error was gone.