What can make a shofar non-kosher?

Question

I had always assumed that any shofar made from a ram's horn is kosher without further investigation, but this question asks about validating the kashrut of a shofar, including this:

It seems that there is growing concern, and perhaps even some evidence, that Shofar producers are selling Shofaroth with very well disguised defects that render them not Kosher.

What kinds of defects can render a shofar not kosher? Are any of them things that can develop over time, such that I should be checking a shofar before using it?

(Note: the other question asks about verifying the kashrut of a shofar; my question is about the kashrut requirements themselves.)

Answer 1

Maimonides in the first chapter of the Laws of Shofar, Sukka, and Lulav, in halachot 5-7, lists several instances such as punctured or cracked shofars that render it non-kosher.

So, to more directly answer your question, one could at one point have a perfectly kosher shofar, and then, at some later point in time, it becomes invalid for use on Rosh Hashana because it becomes punctured in a specific way. (This can happen very easily if someone mishandles the shofar, as you can imagine.)

See here for the text, and here for an english translation and commentary.