[2019] Very cool

This is a known issue but one that's quite tricky to nail down, the devs are still working on a fix. For now if you run into this issue - you only need to initiate conversation with the Museum Guard who stands by the cape in Varrock Museum, you don't need to buy the cape at all.

Talking to the guard basically runs a check on the player to see if they qualify for the cape, which is why this 'fixes' itself. Hopefully we can get a fix out soon :)