Mdthbs

The prefix in- can mean "in" or "into" or similar, as in inire.
It can also mean "non-" or "un-", as in infelix.
Both meanings of the prefix are attested, but I am not familiar with any case where both readings of in- are possible.
Is there a word with which both meanings are attested (or otherwise reasonably defensible)?

This question arises from this earlier one and especially comments to the answer by Draconis.
I am looking for something like illapsus meaning both "in-fallen" and "non-fallen".
I imagine participles, especially those that have become adjectives in their own right are a potential source of such ambiguous prefixes.

It is worth pointing out that native speakers of Latin were well aware of the ambiguity referred to by Joonas in his question (directional/locative prefix IN- vs. negative prefix IN-). For example, consider the ambiguity of invocatus ('called upon' and 'not called upon') that is comically exploited by Plautus in the following text (Pl. Capt. 1, 69ff.):

Iuventus nomen indidit Scorto mihi,

eo quia invocatus soleo esse in convivio.

Scio absurde dictum hoc derisores dicere,

at ego aio recte. Nam scortum in convivio

sibi amator, talos quom iacit, scortum invocat.

Estne invocatum an non est? est planissume;

verum hercle vero nos parasiti planius,

quos numquam quisquam neque vocat neque invocat,

quasi mures semper edimus alienum cibum;

For relevant discussion of the ambiguity of invocatus involved in 'called upon' (directional IN- + vocatus) and 'not called upon' (negative IN- + vocatus), please read the following note 1 contained in this English translation (The Comedies of Plautus. Henry Thomas Riley. London: G. Bell and Sons, 1912).

1 Because invocated: "Invocatus." The following Note is extracted from Thornton's Translation of this Play:--"The reader's indulgence for the coinage of a new term (and perhaps not quite so much out of character from the mouth of a Parasite) is here requested in the use of the word 'invocated' in a sense, which it is owned, there is no authority for, but without it no way occurs to explain the poet's meaning--which, such as it is, and involved in such a pun, is all that can be aimed at. The word 'invocatus' means both 'called upon' and 'not called upon.' Ergasilus here quibbles upon it; for, though at entertainments he attends, as it is the common character of Parasites to do, without invitation, that is 'not called upon;' and as mistresses are 'called upon' that their names so invoked may make their lovers throw the dice with success; still, according to the double sense of the word, they may be compared to each other, as they are both, according to the Latin idiom, 'invocati.'"

NB I: A couple of ambiguous words like the ones commented on by Tom Cotton and cnread are: oratio inscripta est ((I) NEG in-: 'the speech is unwritten' and (II) DIR/LOC in-: 'the speech was inscribed/signed' (e.g., with the author's name)) & inauratus ((I) NEG in-: 'not ornamented with gold' or (II) DIR/LOC in-: perf. pass. participle of inaurare 'to cover with gold').

NB II: I think it is also interesting to point out that in Latin there are very few examples of co-appearance of the negative prefix IN- with the directional/locative IN-. For example, the three ones found in Baldi (1989: 6) are: ininventibilis, ininvestigabilis, and ininvicem. Cf. this link for a more complete list.

I'm inclined to agree that participles are a likely source, as you suggest.

An example that springs to mind is innatus. As the perfect participle of innascor it means 'having been born in', etc. A little surprisingly, as that of innato it would mean something like 'having been floated upon' (perhaps, for instance, hydrargyrum innatum est ferro, though I can't find a proper attestation, and Latin might better express this the other way round).

On the other hand there is innatus, used adjectivally by Tertullian — Innatus deus; an non et innata et materia? (Lib. adv. Hermogenem) — which appears to mean 'not born'.

There is also the pair intego, intectum, 'cover over' and intectus, used adjectivally for 'not covered'.

I imagine that a search for further (and maybe more convincing) instances could be tedious, but these may suffice to indicate that the two-readings possibility exists.

The example that comes most immediately to my mind is invisus.

As the perfect passive participle of the verb invideo, it means 'looked at askance' (i.e., looked upon, but in a bad way), and it's regularly used, by extension, as an adjective meaning 'hateful' or 'unpopular.' Use of this adjective/participle is very common.

As the negative of perfect passive participle visus, from the verb video, it means 'unseen.' This is somewhat rarer, but OLD cites examples from such sources as Cato, De agricultura 141.2, Cicero, De haruspicum responso 57, and Apuleius Metamorphoses 5.3.