As the issue tracker uses Creole markup in same fashion as Wiki, this may be a wiki bug?
According to Bitbucket markup documentation, nested lists are supported, yet I cannot achieve nested lists within Issue Tracker. I have not tested Wiki for this ability.
Nested list not working ex: Item * Sub item
Numbered sub item
EDIT: Ahhh... So the error is in the live preview, as the nested list renders properly after submission. That was giving me hell last night...