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When you select a hero in spectator mode a little bar opens up in the lower left with their items and stats. The health and mana bar have a white line running through the center of them that makes it very difficult to read the numbers, although it's much less severe for the mana bar I guess because white numbers don't blend into a blue background as much.
A similar white line also exists in the new in-game hud although it's not as bad there. It is still needlessly obscuring though.