I've flown in an aeroplane with an under-reading tachometer. As I recall, the first 50 hour check they ordered the part. The 2nd 50 hour check they found they had ordered the wrong part. Eventually of course it got done, but it took a while through no real fault of the school.
It was no biggie but perhaps meant that some of the numbers took longer to stick. I can't see anybody would advocate sending a student on their first solo with a dodgy ASI though.