I think you're referring to "the G string problem". The deflection of the third string at the first fret G# pulls the string a bit sharp relative to the open

I think you're referring to "the G string problem". The deflection of the third string at the first fret G# pulls the string a bit sharp relative to the open G on the third string. There is a specialized guitar nut called the Earvana that has additional material to shorten the open G string a wee bit to compensate for this. In real life, getting any (non-keyboard) instrument to actually play in tune is the hardest part.