I had posted about this a while ago on my blog but I'm still pretty interested by it:
Why does ABBA the Museum have a different variation of Agnetha's blue 1979 costume on display instead of the one she's seen wearing in the majority of the pictures from the 1979 tour? (please excuse my inability to coherently explain my thoughts. )
The one in ABBA the Museum has one shoulder strap while the one she's usually wears has two.
The only time I've seen Agnetha wearing the one shoulder strap one is on this picture from the Japan leg of the tour.
Maybe it's the same costume just she had it modified later by cutting off the right shoulder strap? It does look a bit like she just tucked it in, but that would be a little uncomfortable. I'm sure there's a reason for removing/doing something different with the shoulder strap, I just want to know what it is. xD