Are the expressions "mirabile visu", and "horribile dictu", in the ablative case? If so, shouldn't it be "mirabili visu", and "horribili dictu"?
Never forget about meaning, without which an expression turns into a collection of separate words. Your question presumes that the two words form a noun-modifier phrase mīrābilis vīsus "a strange, fascinating sighting", horribile dictum "a horrible saying", put in the Ablative case. If this were the case, the ending would indeed be wrong in the Latin of the classical period, or generally of antiquity, where -ī is standard. That said, -e becomes quite common towards the medieval period following the collapse of the Accusative with the Ablative, and it can be occasionally seen in the medievally-tinged Latin of the Renaissance.
However, as soon as you recall that the actual meaning of these expressions is "strange to see" and "hair-raising to say", you will realise that you aren't dealing with a single noun phrase in the Ablative, but with two separate phrases, a head and a complement: mīrābile < mīrābilis "strange" and vīsū < vidēre/vīsus "to see/to the sight".
- The first word is the head, formally a neuter adjective in the Accusative that can functionally be an adverb (the so-called 'adverbial Accusative'), e.g. mīrābile sonāre 'to sound pleasantly unusual'. This is also the case here - in fact the whole phrase is a parenthetical sentence disjunct, just like in "And then - to my horror! - I saw...". At first blush the adjective proper can be in any grammatical case that its function requires (e.g. Accusative in mīrābilem vīsū effigiem "a most remarkable, uncanny image/likeness"). However on practice it never seems to be used in the Ablative or the Dative precisely because this would result in a double-reading as a single noun phrase (Garden-path sentence). This is therefore very strictly avoided.
- The second word is a complement dependent upon the first and completing its meaning. This form is known as the supine (here's a friendly article on these by Latinitium), and the one we have here is doubly problematic:
- Firstly, it can be said to be both nominal "to the sight" and verbal "to see". Evidently it moved from the former to the latter category in the course of time.
- Secondly, its case superficially appears to be Ablative (and many a school grammar will tell you the same), but its meaning clearly points towards the Dative (which is the origin of the Infinitive in many other languages incl. English); moreover, the Latin found outside of school grammars actually had a Dative in -ū as an alternative and seemingly more original form, which makes the latter explanation by far preferrable.
- See this question for more discussion on both these points.