Ladders are useful - but they also cause a lot of misery. According to the CDC and NIOSH data 500,000 people receive medical treatment, and more than 300 people die, each year from ladder-related falls (https://www.cdc.gov/niosh/topics/falls/mobileapp.html).
Because extension ladders reach higher than step ladders, you hit the ground harder and the injuries are often more severe.
Forewarned is forearmed. If you know why people fall from extension ladders then you can make sure you are not one of them. Here are 10 reasons:
1) Ladder not tied off. By tying a rope at 45 deg from each side of the ladder to the house, you can stop the ladder sliding sideways. For example, if you commonly use a ladder to unclog a problem downspout, then install heavy duty stainless steel shoulder lag eye bolts into the vertical 2 x 4" or 2 x 6" wall studs through the siding on either side of the downspout. Tie the ladder off to the eye bolts before you start working on the downspout - obviously, use a knot that cannot slip. (At 45 degrees the eye bolt can only safely withstand 30% of its rated capacity; for example, at that angle a 3/8" bolt can withstand 380 lbs of force.) When you buy the eyebolt make sure its eye is formed from a complete and continuous loop of metal, not just a partial ring, otherwise it could bend open and fail to save you.
2) Foot slipping off the rung. Don't hurry. It is well known that there is a speed-accuracy trade off when humans make any kind of movement. That is, the more you hurry, the less accurate your foot placement will be. Yet your stability depends completely on having at least one foot solidly on the rung. If your foot slips off the rung, it is doubtful you have the hand strength to save yourself (if you don't believe me, see some research we did on the topic https://www.researchgate.net/publication/41668118_Hand-Handhold_Coupling_Effect_of_Handle_Shape_Orientation_and_Friction_on_Breakaway_Strength).
3) Carrying something heavy up the ladder in one hand. OSEH recommends using 3 point control when climbing a ladder - that is two hands and one foot or two feet and one hand should be in contact with the ladder at all times. However, people almost never use 3 point control. They climb using opposite hands and feet - so they only have two points in contact with the ladder, right hand and left foot, or vice versa. If the foot you are bearing weight on slips, you will start to fall, and it is doubtful you will be strong or quick enough to save yourself.
4) Base of ladder slips while you are on the ladder. You want to clean a gutter or replace a roof tile, so you lean the extension ladder against the gutter with its feet on wet wood decking or planking. As you climb up the ladder the base of the ladder suddenly slips backward on the wet wood. To save yourself you instinctively grab the rim of the gutter through the space between two rungs. If the gutter doesn't simply fall off the house with you holding on to it, the upper rung will trap both your hands against the gutter and stop the sliding. But you are now completely stuck with your hands trapped under the rung against the rim of the gutter - if you pull them out the ladder will again start to slide backwards and you will certainly fall. Moral of the story: Don't ever trust wet wood. The base of the ladder should always be tied off on either side to the house to stop it slipping. Or use long deck screws to temporariy fasten a 2x4" on the decking behind both ladder feet, so they simply cannot slip backwards.
5) Using too cheap a ladder. There are five classes of extension ladder, Class III (light duty, 200 lbs), II, I, IA and IAA (Extra Heavy Duty, 375 lbs). Light duty ladders are designed for lighter individuals and are made with thinner materials (aluminum or fiber glass). Unfortunately, they vibrate more from side to side when you move on them than heavy duty ladders do. You can inadvertently start this side-to-side vibration by making a sudden movement or by trying to pull something heavy like a branch that suddenly gives way. If you are older or have bad balance you may be unable to stop the vibration, lose your balance and fall.
6) You fall as you are trying to get on or off the roof from your ladder. Even if you have extended the ladder several feet past your roof line, it is always awkward getting onto the roof from the ladder or getting back on the ladder from the roof because you have to step around the ladder. If upper part of the ladder is not tied off, it can slip sideways as you try to step around it leaving you with few options to save yourself. They make step through ladder extensions do you don't have to step around the ladder - see https://www.northerntool.com/shop/tools/product_200661617_200661617?cm_mmc=Google-pla&utm_source=Google_PLA&utm_medium=Construction%20%3E%20Ladder%20Accessories&utm_campaign=Guardian%20Fall%20Protection&utm_content=50921&gclid=EAIaIQobChMIvrXw3ZDM4gIVpf_jBx36VQR9EAYYBSABEgJl2_D_BwE
7) You haven't been on a ladder for a while. Maintaining any skill takes practice, whether playing an instrument, balancing on skates or using a ladder. Even if you grew up climbing ladders for an occupation, if you haven't been on a ladder for a while, you are no longer as skilled as you once were. In addition, if you are older, you are not as quick or strong as you once were. Finally, as we get older we acquire impairments like arthritis, painful joints or nerve damage that limits our ability to make certain movements quickly and accurately. All of these factors can slow and weaken your balance recovery response when most needed - and the fall starts unchecked.
8) Ladder set up at too steep an angle. The side of the ladder has a diagram showing the angle you need to incline the ladder at when you set it up against the house. If you set it up with too steep an angle you can risk falling backward with the ladder, especially if you have something heavy on your back. If you set it up noy steep enough, the ladder is more inclined to slip backeward when you climb it.
9) Branch falls on you. You have the ladder against a tree and are successfully trimming the base of branches above you when one which by rights should have fallen beside you unexpectedly rolls off other branches which entangle it on the way down so that it drops right on top of you. Unless you hold on tight it can sweep you right off the ladder.
10) Ladder moves unexpectedly. Let's say you are taking a tree down and you have taken all the branches off as far up as you can reach. Now you simply have to cut the trunk above the highest rung on the ladder and have it fall away from you. The problem is that once it is cut , cracks and starts to accelerate away from you, its base will unexpectedly push back on the ladder moving you suddenly backwards when you least expect it. Hopefully you have fast enough reactions to save yourself.