Sorry for the title, I couldn't find a better way to explain it. So let me try here. After years of MG playing MG I just realized that workers mining gold, if told to start mining stone (or vice versa) they don't immediately follow your order. At first I thought it was a bug but then after more observation I realized that it will eventually do it. I think this is counter-intuitive and a bit annoying, especially if you are in a frantic part of the game.
--What happens now--
1 worker is mining gold
2 you tell it to mine stone
3 worker keeps mining gold until he reached his maximum load
4 transport it to the base
5 starts mining stone
--What should happen-- (which is what happens when a worker harvest wood and you tell it to start mining)
1 worker is mining gold
2 you tell it to mine stone
3 worker stops mining gold
4 transport it to the base whatever amount it has harvested
5 starts mining stone