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Dynamic rollover is worst with the right skid on the ground (counter clockwise main rotor) and with a crosswind from the left, with left pedal applied and thrust about equal to the weight (i.e. hovering). A machine can roll upslope if you apply too much cyclic into it, or downslope if you apply too much collective, enough to make the upslope skid rise too much for the cyclic to control. Avoid it by keeping away from tail winds, and landing and taking off vertically.
Ground Effect
In the hover, downwash is stopped by any surface within about 1 rotor length. Because downwash velocity is reduced, so is the lift vector, which becomes more vertical (which itself increases thrust a little more and reduces drag), resulting in less induced drag and a reduction in the power needed to hover (a Bell 206 typically will need 15% less in the ground cushion). In addition, the accelerated air, having slowed down, increases the pressure underneath the rotor disc.
The effect is even more pronounced when you lower the collective to stop climbing, and will be more apparent closer to the ground. Factors that will reduce this are the surface you are hovering over (the harder and smoother the better, and the more level), and the wind, which will vary the direction of the downwash from under the blades. Any above about 10 kts, of course, will produce translational lift.
Recirculation
When hovering near the ground, some downwash comes back on itself and goes through the rotor disc twice, which reduces lift whenever this happens because it does so at a higher speed and reduces the space available for the angle of attack in the resulting vector. Vortices are present at the rotor tips all the time, because they are caused by centrifugal force (fanjets enclose the ends of the blades to stop this), but they are usually more than offset by ground effect.
If your ground effect is reduced for any reason (see above), the chances of recirculation increase, requiring more collective and cyclic to compensate. Where the downflow is actually prevented from escaping properly, as when hovering close to a building, or in a tight confined area, the effect will be to tilt you in a direction 90° from where recirculation was introduced, or even pull you down if all sides are affected (as when landing in a courtyard). Thus, if you are hovering a 206 next to a building in front of you, the recirculation occurs at the front, but the disc will tilt to the left and make the left skid hover lower than usual which, if it catches you unawares, might cause dynamic rollover (see above). If you are closer than a third of your disc diameter, the advancing blade is also affected, in the above example, pulling you towards the building.
Be particularly careful within 1 rotor diameter of another helicopter.
Vortex Ring
This occurs when you encounter your own downwash, and you don't need a high rate of descent to do it (in essence, the vortices that should trail after you in the cruise remain around the machine at low speeds and interfere with your lift). The symptoms are random vibration, buffeting, pitching, yawing, rolling, an accelerated rate of decent and momentary loss of cyclic control. It is caused in a similar way to recirculation, since the airflow caused by the descent increases the blade vortices, which reduces the angle of attack as they share the same space. With a rate of descent matching the speed of the downwash, there is no angle of attack, and therefore no lift (the root area will be stalled). If you like, imagine the outer part of the main rotors encased in a large doughnut of recirculating air:
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