You'll want to look into turn 55 of
this replay. Specifically, Ferroseed's HP bar is displayed as 0%, after the opposing Azelf faints from Leech Seed damage, and it stays until Ferroseed really reaches 0%.
This is a very good tool, though. It's much easier to show replays now because of this.
EDIT: In
this DPPt replay, Porygon-Z's damage is shown as 0% after sponging another Hydro Pump from Rotom-W, even though it survived (turn 25). Same thing happens next turn (turn 26) when Nidoqueen uses Earth Power on Lucario, it does 79% but then displays Lucario's HP bar as 0%.
Also, if you click on "Go To Turn" and type in 0,
the screen will be blank and say "seeking" in the middle, while the log is unaffected and continues to roll out.
Finally, if you click on "Skip To Next Turn" rapidly, the sprites and Pokemon names from the next turns will
overlap on each other. You can recreate this if you start up the replay anew, and click the "Skip to Next Turn" button rapidly. It will then show
overlapping sprites and names, and will freeze. Other glitches have sprung up from this one: one time, it said "seeking" in the middle of a blank screen as if you specified turn 0, and then everything was normal after, and another time,
Forretress lacked his HP bar.