Occasionally, people who are about to have a seizure experience warning signs leading up to the event. Shinnar explained that this is more often a characteristic of a partial seizure, which starts in one side of the brain and later spreads to involve the entire brain.
Hours, or even days, before one experiences a full-blown seizure, he or she may experience a prodrome, commonly referred to as an "aura." This phase often involves anxiety, irritability, difficulty concentrating or sleep disturbances.
In general, seizures like the one experienced by Roberts can be broken down into three phases.
The first phase, known as the tonic phase, involves a contraction of the muscles of the back and arms, leading to a rigid posture and increase heart rate and blood pressure. This phase can last between 10 and 20 seconds.
The next phase is known as the clonic phase and encompasses much of what is traditionally associated with seizures -- rapid convulsions and a clenching of the jaw muscles.
Though the seizure sufferer may appear to be in pain, they are actually completely unaware of their condition.
"When you are in a general convulsive stage, everyone around you may be scared, but you are not conscious," Shinnar said. "You don't feel anything."
During the final phase, known as the postictal stage, the person who has experienced the seizure will gradually regain consciousness.
"After a seizure, one will basically have a bad headache, kind of like a hangover," Shinnar said. "Basically, you are washed out. Your brain has run a marathon, and it is tired."
But even though those who experience seizures may not worry at the time of their episode, they may live in fear of the implications of their condition -- particularly the danger of harm to themselves or others.
"Very prolonged seizures can cause damage, if they last on the order of 20 to 30 minutes," Shinnar said. "But a seizure that's brief is basically not in and of itself considered dangerous; the main risk of a brief seizure is being in the wrong place at the wrong time."
This could be behind the wheel of a car, at the top of a staircase -- or even by a placid lakeside, where drowning is a possibility.
While physicians can be fairly certain as to what Roberts went through during his episode, the exact causes for his seizure remain unclear.
"In many cases when seizures occur in older adults, we never find out the underlying cause," said Dr. Jerome Engel, director of the Seizure Disorder Center at UCLA.
"You can reconstruct it," Erba said. "But it is so important to do good detective work when you do a history. You have to be imaginative."
One possibility is that the sun reflecting off of the lake close to Roberts could have created a strobe effect, setting off what is known as a photosensitive seizure. Another possibility is that extreme stress or lack of sleep in preceding days may have led to the event.
The results of Roberts' electroenchalography, or EEG, tests will likely help determine what might have brought his most recent seizure about -- and whether he will require treatment to avoid future episodes. Fortunately for Roberts, serious potential causes such as tumors and stroke have been largely ruled out by tests.
Regardless of the cause, Grisolia said the incident highlights the fact that seizures are a common -- and often controllable -- problem.
"We have people as high-functioning as Justice Roberts and other people who are really quite impaired," he said. "There is a big spectrum in terms of the kinds of people who can have [seizures]."
ABC Medical Unit Researcher Dr. Sharon Bord contributed to this report.