1. Blender gets stuck on something.
2. They pull it out of the mixture, and hold it in one hand.
3. They stick their fingers in with the blade to clear the blockage.
4. The blender shifts, their hand presses the trigger, the blades spin.
5. ER visit.

There's been some writing about this, and the general advice to avoid this is to unplug the blender before clearing the blade. Which works, but is annoying enough that people don't consistently do it. On the other hand, most consumer ones come apart into two pieces for cleaning. There's the motor end, and the blade end:

If you take them apart, there's no way for a slip of your hand to pass power through to the blades. So here's a useful rule: always keep the parts separated unless the blade is in something you want to blend.

Following this rule, the scenario above would become:

1. Blender gets stuck on something.
2. Separate the two halves.
3. Pull the blade end out of the mixture, hold it in one hand.
4. Clear the blockage with the other hand.

You can still cut yourself on the sharp blades if you slip, and cleaning it another way so you don't put your fingers in it is still better, but the blade isn't going to spin under power with fingers inside.

(Some new immersion blenders have a safety lock, but these are very unpopular among reviewers. They they make it hard to impossible to use the blender with one hand, and if you're using both hands on the blender you don't have a way to hold the bowl or cup.)