Shin, for Ashkenazim, has to have a pointy bottom. But Sephardim don't necessarily agree with that, and many Sephardi styles give shin a rounded or flat bottom. Now, most Ashkenazim don't think that this is a deal-breaker; you can still recognise the letter as shin, after all, but a few Ashkenazim do think it's very much a deal-breaker. They may even avoid Torah readings from a Sephardi-style Torah on this basis. Some Sephardi scribes add a nominal point to their shins, as here, for compatability:
This is a formalised example of how minor variation in letter forms can affect how kosher it is - formalised because the variation is accepted as valid by different branches of the tradition. Accidental variation is more likely for the sort of proofreading I'm doing. A more common example, of ambiguity affecting kashrut, follows shortly.