Had some pretty bad beer in my time. 'Ba-me-ba', Ba-Muoi-Ba, or Biere 33, was pretty bad in Vietnam, and they said it was 1/4 formaldahyde. Even worse was Biere Larue. I had some of the latter, warm, over ice, out in the field once, and after the ice melted there was a half inch of sludge in the glass, either from the beer or the ice.
But the worst beer I ever drank was long after hours in Juarez, in the 1950s when it was safe to go there. My friend and I went up some dark alley, with a lit 'Cerveza' sign at the end of it. I could never find it again in the day. We entered and asked for Dos Equis. No, they didn't have it. Corona, or Cruz Blanca; no never heard of it. All they had was something called Chihuahua Beer, which tasted like it came out of the dog of that name. Never again.