Abby identifies the shell casings at the crime scene as coming from either a GLOCK or Beretta. The striker from the GLOCK and firing pin of the Beretta each leave a distinctly different looking mark on the primer of the shell casing. A forensic scientist (especially one of Abby's ability) would not confuse the two.
Tony DiNozzo chides Ziva David for not assimilating into American culture because she doesn't use contractions in her English. However, in the scene with the vending machines, she does use contractions. (She does in other episodes as well.)