If you notice a greasy buttery layer forming on top while the toffee cooks, it means the toffee is separating. You can try adding in 1 tablespoon of water at a time and slowly stirring to recombine. Separating (and/or crystallizing where the toffee turns sugary and grainy) can happen with sudden drops or spikes in the temperature or with constant, vigorous stirring. It can also happen if the sides of the pot are scraped, reintroducing crystallized sugar pieces.