If you accept unicode for strings that should be "unique" (eg username), there are various normalization schemes that basically convert equivalent-ish looking characters into a consistent hash.
Followed by spam filters keying in on use of such alternative/modifier characters and marking them as spam. Better check your spam folder if you get a lot of e-mails with mathematical formulas.
This is called a Homoglyph attack.
If you accept unicode for strings that should be "unique" (eg username), there are various normalization schemes that basically convert equivalent-ish looking characters into a consistent hash.
I have no doubt spam filters use this.