This isn't a bug or glitch, it belongs in either questions about scratch or help with scripts.

Here's the scratch wiki article on the pen extension: pen extension

Basically, to get the pen blocks in your project, click the add extension button and select pen.
The way pen works is that it allows you to draw lines onto the stage.
When you put the pen down, it starts drawing, and when the sprite moves it draws a line behind it.

You can also stamp, which stamps an image of the sprite onto the stage.


There are also many blocks that change the style of the lines drawn.


An explanation of each block:

erase all::pen

This removes all pen lines

stamp

Stamps an image of the sprite to the stage.

pen down

This makes it so the sprite starts drawing.

pen up

This makes the sprite stop drawing.

set pen color to [#778f50]

Changes the color of the lines drawn by the pen.

change pen [color v] by ()::pen

set pen [color v] to ()::pen

Sets/changes the color, brightness, or saturation of the lines drawn by the pen.
Color is what color it is (0=red 33=green 67=blue, numbers in between are mixes)
Saturation is how strong the color is (0=no color visable 100=full color)
Brightness is how bright the color is (0=black 100=bright)

change pen size by ()

set pen size to ()

Change/set the width of the lines drawn by the pen.

you can use the pen, by acting its a a sprite for example:

turn ccw () degrees

pen down
pen up