Do you have a machine with an E61 brew group (such as models made by Rocket, Quick Mill or Izzo) that sometimes pulls bitter or burnt shots? It could be that you’ve left the machine on for an extended period of time and that the temperature is too high to pull an ideal shot.
We recommend pulling the lever and letting some water run through for a few seconds before you place your portafilter in the brew group, which will release some of the steam pressure and cool the temperature down a bit. If the machine has been sitting unused for more than 10 minutes, be sure to do this and your shots will taste great every time.
(E61 diagram courtesy of HomeBarista.com)