Weaknesses in the ice are often under bridges and overpasses. Places where water is discharged will freeze later or less.
Plant material rotting on the bottom can occur methane bubbles freeze in the ice. This weakens the ice. This (in Dutch called:) crack ice is dangerous.

Here, too, care and prevention is the most important.
Do not just skate alone, so who is with you is an alarm and / or help.
Do not go skating in the dark or at dusk, you will not recognize weaknesses and gaps.
Skate therefore never in the (covering) snow.
Take a mobile phone. In a waterproof cover.
Take a rope along, and one or two bodkins (with protection, e.g. a cork on the point).

If you fall through the ice, or in a hole, then try to come out asap. The literally breathtaking cold bites through your skin. Hypothermia takes control over the muscles quickly.

Break toward the shore weak ice at the edges as much as possible until you get to thicker ice. Try to lift with both hands yourself till your waist, and then slide forward over on the ice and roll. Do not stand, but distribute weight and pressure by sliding or crawling. This also applies to those who come to help. Do not go together, but stay widely spread.
Or better yet, put ladders or shelves on the ice to walk on.

Another way to get out of the (small) hole is to lie with your back against the ice edge and put your feet to the edge ice you are facing. Press yourself up (on your back).

If you want to help someone out of a hole do not go or stand on the wet and slippery edge of the ice, but go lie on a distance. Use long branches, or belts.

If you get under the ice, note the color difference to find open water. If there is snow on the ice, then the ice is darker and the hole lighter. Otherwise, is clear ice lighter, and the moving water of the hole darker.

With an awl, you can pull yourself out of water. Save the awl in the ice as far away from you as handsome.

If you 're out of the water, it is important to keep you warm. Keep moving, quickly pull dry clothes, take a warm bath or warm up by body heat from others.

To step as safely as possible on slippery  ice walk like a penguin: small steps and flat feet, where the center of your body weight every time stays above your standing foot.