Along the white lines, a third line suddenly appears and tangles itself into another (the two short white lines, upper left).
This one looks like a #toughie because of the line appearing and not quite forming a vertex.
Because of this, you might mark it as "Something Weird". From the FAQ:
Q: What qualifies as "something weird"? - A: @andy.haas: "Lots of tracks in one tight bunch, or coming out at weird angles but not forming a vertex, or a big amount of tracks on one side but the other side, or lots of energy in the calorimeter (the red/green areas beyond the tracker), or things in the muon system (the outermost detector)... what ever looks weird to you!"
However, if you come back to the discussion page for this object and select the Slice view option, you'll see that the three white lines are shown all together, and they run the entire length of the image; the same object looks different between the Normal/Zoom and Slice views! Since you're unable to select which image you're classifying, just mark it according to what you see in the classification screen.
Edit: Thank you for mentioning this! I've updated my comment in the discussion page to make a note of what you spotted.