per this answer
The bits at the top and the bottom that tuck in to seal the box are called the flaps.
what is the gap/crack/flaw pointed out by red arrow shown below (img_1) called?
is the part that is labeled blue shown below (img_2) also a slot?
Another concern is the hole/slot in img_1 is hollowed out while the one in img_2 is not.
The feature indicated in img_1 would most likely be called a hole or slot. It could be called a gap as well, but that is more commonly taken to mean the space between two objects or features rather than a space or cut-out inside an object.
The blue circle in img_2 surrounds something that could be called a gap or slot. Calling it a hole is less suitable because it is a space between two objects (two pieces of cardboard) instead of a cut-out in a object like the one in img_1, but I could imagine someone calling it a hole anyway.
I agree with James that both features would never be called "flaws" (because they were intentionally designed as part of the boxes), and I would also say that "crack" is unsuitable. In the technical and common definition of the term, a crack is something that forms as a result of mechanical damage, like the one shown below in a sheet of metal. The hole in img_1 was intentionally cut out and did not develop as a result of mechanical stress.
That is a hole. It could also be called a gap, or a slot. These are all generic words. There is no specific word for the "hole in the lid of a cardboard box".
It isn't a flaw, because a flaw is something unwanted.
