Best advice I ever received

Debugging:

"Never, ever assume you think you know where a bug comes from". I have seen developers arbitrarly put in break points and debug statements
and then work there way forwards to the point at which the error manifests. Don't do this, precisely becuase your break point or debug statement *is* arbitrary*.
Becuase the framework or library you're using may have poorly understood or poorly documented, unpredicatable
or plain broken behaviour, then place your breakpoints or add your logging there first and only then work your way forwards.

UX:

"Always assume your user is deaf, dumb, blind and stupid." No offence meant *whatsoever*. If you can get your project to be used by someone of this ilk, you've already won.