I think it depends on what you're validating. If you are trying to validate a name, you had better get it right! For example, my Visa still says Reginald Braith

I think it depends on what you're validating. If you are trying to validate a name, you had better get it right! For example, my Visa still says Reginald Braithwaite-Lee, but I might register on your site as Reg Braithwaite. Did I misspell my name or is that what's on my Visa? Is the hyphen a typo? OTOH, some things seem to be more certain, like rules about check sums. I like the approach suggested by many folks: Use JS to validate on the client, and put up a "Are you really, really sure?" message for things that seem unusual like a single name or a funny character.