Is there a transit around the 19-day mark? The brightness seems to drop in this area, but not in others.
The event is not real it is a simulated transit.
Simulated planet radius: 2.68 Earth radii.
Simulated planet period: 683.262 Days.
As Martti pointed out this is a simulated transit.
So Yes there is a transit visible at the 19-day mark, but in this case this is a simulated transit the science team injected into a real Kepler light curve- you can see the label on the bottom left on the Talk page says simulated and a message should have popped up when classifying saying it was a simulation. Simulated planets help us understand what the true frequencies of planets are by telling us what Planet Hunters as a collective can find and can't find. This way if we know we can find Jupiter-sized objects but only 80% of the simulated transits, we know there's 20% we're missing.
We always put up a message saying the light curve was a simulation after classifying and tag them in Talk, so it's clear to everyone. We can't say before because they might change how you or other classifiers might mark the curve. If you knew there was a simulated transit in the light curve, you might me more likely to mark an iffy feature than you would if you didn't know that.