The Bathroom
The first time Ed walks in on Winry in the shower it is a complete accident. He had no idea what the problem was or why he got in trouble for it.
The second time Ed really needed to use the toilet. Winry was scarred for life.
The third time he had transmuted something really advanced. After looking for her all over he was so happy to finally find Winry that the fact he wasn't supposed to barge into the bathroom went completely over his head.
The fourth time was because he thought it had been Al. See, Al liked to play with toys in the tub and seeing it firsthand would have been instant blackmail.
The fifth time was because Ed believed something was dying. He personally thought a cat. To the embarrassment of both of them it was just Winry singing.
The first time Winry walks in on Ed in the shower is on purpose.