Complex sentence: When users are able to choose their personal display mode, the hierarchical structure can easily be disrupted and become unusable: users of the flat or linear mode may append their reply to the most recent post by default, regardless of what post is actually replied to.
Split into simpler sentences: When users are able to choose their personal display mode, the hierarchical structure can easily be disrupted and become unusable. Users of the flat or linear mode may append their reply to the most recent post by default, regardless of which post is actually replied to.