The One With The Cake - S10-E4
Corrected entry: In The One With The Cake, Chandler says that he has to go to the bathroom and Phoebe and Joey run in after him because they say he's trying to escape out the window, but The One With The Fertility Test we see a glimpse of the left wall (the only one we rarely see so may have a window) many times, from the kitchen perspective. There is no window, just things hanging on the wall, possibly a towel on a hook.
Correction: In other episodes, when the shower curtain is opened, you can see the window is on the back wall on the far side of the tub (which is not uncommon).