²For example, the (lower-case) German “ß” doesn’t have a unique upper-case equivalent: “ß” usually maps to “SS” (for example “groß” $\leftrightarrow$ “GROSS”), but if that would conflict with another word, then “ß” maps to “SZ” (for example “maße” $\leftrightarrow$ “MASZE” because there’s a different word “MASSE”). Or at least that’s the way it was prior to 1998. The 1998 revisions to German orthography removed the SZ rule, so now (post-1998) the two distinct German words “masse” (English “mass”) and “maße” (“measures”) have identical upper-case forms “MASSE”. To further complicate matters, (the German-speaking parts of) Switzerland have a slightly different orthography, which never had the SZ rule.

French provides another tricky example: In France “é” $\leftrightarrow$ “É” and “è” $\leftrightarrow$ “È”, whereas in (the French-speaking parts of) Canada there are no accents on upper-case letters, so “é” $\leftrightarrow$ “E” and “è” $\leftrightarrow$ “E”.