Navy Blues - S7-E21
Corrected entry: Jack goes into the kitchen with Janet to get "Jim Bob" Furley a glass of brandy. When Jack returns from the kitchen to hand Furley his drink, Jack trips and spills the drink all over Furley, causing Furley to give the Partnership to Jack's Navy Pal, Bill. Furley then excuses himself and heads to the bathroom to clean up. But Furley isn't supposed to have met Jack before, so he wouldn't know where the bathroom is. He goes right to it anyway without asking where it is. Lucky Bill didn't notice.
Correction: Also, considering usually the bedroom doors were left open in most episodes and Jack came out of the kitchen with the brandy, so the only other door would be the bathroom, so it would even be easy for even a stranger to figure out.
Correction: As you state, Bill didn't notice, so it was just a bad decision by a character, not a mistake. But it's also possible Bill did notice and figured "Jim Bob" guessed correctly or knows it's not that hard to guess which door is the bathroom in a small apartment.
Bishop73