The waves
@Will Gilmore describes are often referred to as Rogue waves. There has been a lot of research on them over the past few years and they are more common than was thought. They have been found to occur all over the ocean and at varying sizes, measured in a few feet to tens of feet. The waves are often square and steep sided. We get them a lot on Lake Ontario due to shifting wind patterns as fronts roll through. A wave pattern gets started, the wind shifts and starts a new pattern before the old pattern has subsided and square waves pop up.
One way boats capsize is surfing down a large wave, the bow buries in the trough which stops the front of the boat while the back of the boat continues forward, this causes an abrupt turn and places the boat broadsides to the wave into a broach. The boat then rides up the wave sideways down the backside of the wave, unless the wave is about to break. If it is tall enough and steep enough the vertical lift can roll the boat past the point of vanishing stability, at which point the boat will capsize.
Perhaps more important than wave size is hull shape. Some shapes are better at recovering from a broach or capsize and others are not. The worst boats are catamarans, they are a stable upside down as they are right side up, so it is difficult to right them. Boats with broad flat decks can be more stable inverted. Search for information on the Wing Nutz accident in the Chicago Mac race a few years ago.
Boats with flat bottoms, sharp corners at the chines, and vertical sides offer a lot of initial stability, they don't heel very much at first. However, once they approach the point of vanishing stability they quickly become unstable and will flip. A good example of this are the old aluminum canoes made by Grumman and popular in camps. Step in the canoe and it feels stable and solid, however, lean it over too far and you are swimming, there is very little room or forgiveness. Other canoes, like the Mad River Tripper were very tippy at first, they had a low initial stability, however, they could be heeled over and the gunwale placed on the water and the boat would become very stable. A boat with a rounder or V-shaped bottom will be more resistant to capsizing than a boat with a flatter bottom and more vertical sides. One of the Mac 26s that double as a power boat are good example of a boat with high initial stability and low final stability. Boats like Bob Perry's Valiant 40 and various Pacific Seacraft design have rounder bottoms and softer chines, these boats have good offshore reputations, they heel quickly, but become more stable as the heeling increases.
I guess the point of this rambling is to answer the OPs question, "Real Chance of Capsizing?" with a big "well it depends" on wave size, wave pattern winds, hull form, and crew skill. In general I'd be much more concerned about getting pooped with the companion way open and filling the boat with water, breaking some critical piece of rigging, or blowing out a sail than capsizing.
EDIT: Forgot to add a link to this article on stability.
Boat Stability