Wrong numer of verteces in vertex array

[Guardian]

I'm reading the tutorial on sf::VertexArray ( https://www.sfml-dev.org/tutorials/2.4/graphics-vertex-array.php ) and I don't understand what happens if I try to draw a vertex array with a wrong number of vertices for his primitive type. 
For example: what could happen if I drew a vertex array of 5 vertices using the sf::Lines primitive?

[eXpl0it3r]

In my opinion you're asking the wrong question. You don't need to know what happens, but you need to find a way to prevent that it does happen.

Might be implementation dependent and the GPU driver decides what really happens.

[Guardian]

so it's an undefined behavior and I have to avoid it in every case.

Thank you.