Mentioned this on the episode discussion thread.
The door was, as Stupid_Head not-so-stupidly points out, a relic from the days when Charlie Stubbs lived there. It was very rarely seen even during his time in the flat. It hasn't been seen for years.
I think that the production team have used some big dramatic licence with the reappearance of the door. Another strong example of the 'story' overruling the character, which is something Corrie would deny to their last breath. The story liners, writers and directors really went out of their way to resurrect the door to the balcony, just so that Tina could fall off. Why Tina would go out there at that moment is beyond me. It was also ridiculous that the door was more like one of the ones that leads from the gunnel into the back yards than a proper door.
Artistic licence gone too far, in my opinion.