Seat posts have these little marks on them to remind you not to pull your seat post out too far. They usually have a line, or an arrow or something clear.
Minimum insertion line
What is the danger, you ask? Well have a look here:
These cracks are the result of the seat post not fully inserted in the frame. The real danger is not in noticing your frame is cracking, it is in the frame failing entirely:
The results of this kind of failure are too graphic to put on this family friendly blog, so please mind your seat post insertion marks.