How is it that people manage to sneak onto airport flights?

From my understanding, you can't make it past security, without a ticket, nor a boarding pass, but even if they didn't have a luggage, it would still be impossible, I imagine.

And just where have you heard of people sneaking onto flights?

I think that is extremely rare - at least within the US.
Some people have tried to climb into wheel wells by sneaking onto the runway and trying to jump in at the last moment.

That usually does not turn out well for them - as they usually end up dead from lack of oxygen and/or the extreme cold at high altitudes.

Many years ago, there was a picture in the news that a man took while testing a camera. He took a photo of a plane coming in for a landing. When the film was developed (yes, a long time ago), there was a body falling through the air underneath the plane.

It's not common at all. It's very hard to get through security, but sometimes security is busy and overwhelming and someone can take advantage of that and sneak by.

More often, people have a ticket to one place and mangage to get on the wrong plane, either intentionally or accidentally. Even that is quite rare, and generally requires a signficant lapse in attention and/or a technical glitch.

Simply, no one sneaks on to any flights these days. Security it much stricter and whilst there's the possibility that some may try to board at the wrong gate, the system would confirm they were travelling to another destination. Even someone with a gate pass, which is unique to USA airport, still couldn't sneak on, as they would have no valid boarding pass.

You are referring to Marilyn Hartman… Please read this piece from NPR.

She basically tries to blend in. (Particularly when there's a large family having their travel documents checked.) Keep in mind that this is a very rare events as the other's have mentioned. She still went through the screening process (Body scanner and belongings x-rayed.) despite not having the proper travel documents with her.

She is not specifically a threat as opposed to being nuts with severe OCD involving getting onto an aircraft. In this case the bigger problem is the higher authorities REFUSING to keep her locked up at some facility.

As for people breaching the perimeter fences, that too is rare and is handled by local law enforcement.

Its extremely rare. Out of millions who travel daily, you can count on one hand how many people actually attempted to do this.

There's always a hole somewhere. You have no idea how flimsy some of these airport security fences or areas are. Same for the boarding gate. Just a moment of inattention by the tsa officer and someone could sneak by. A man arguing about his checked bag with the gate agent and someone can sneak by. Of course, they used to always do seat counts before departure but that's not the norm nowadays.