This achievement can't be missed, as it is story relevant.
*** Spoiler - click to reveal ***After Laverne got playable, tell the prison guard you feel pukey. After the doctor leaves the room, get the tentacle chart from the right wall. Get back to the prison, and tell the guard you need to go to the bathroom. Flush the tentacle chart to Hoagie. Switch to Hoagie and have him put the tentacle chart on the table for the flag designs.