The word locus is masculine in the singular, but it can be masculine or neuter in the plural. Geographical places are loca, but places in a text are loci. As far as I know, this is the only Latin word with such behaviour.
Do we know where this "double plural" comes from? Was it always there in the written Latin we know? Is there a similar situation in related languages (Italic, Greek, PIE, or something) that could shed some light on the problem?
As usual, to answer this question we need to step into our comparative linguistics-fueled time machine and go back to Proto-Indo-European times, so we can see what function the ending -a, which we know as a neuter plural ending, had in PIE.
In PIE, this ending -a (or rather *-(e)h₂) did not form plurals, but collectives. A collective refers to a group of objects, but is conceptualized as being a single thing, and for grammatical purposes is generally treated as a singular noun; this was probably the case in PIE, and is why in Greek, for example, neuter plural nouns in -a still take singular verbs.
PIE collectives seem to have been used most often with neuter nouns -- in fact, neuter nouns don't seem to have had a normal plural (at least not in the nominative/accusative), so the collective form more or less stood in for a plural. This is why in Latin and other languages, -a came to be a specifically neuter ending.
But the collective could also be formed from masculine and feminine nouns. Locus is one such example: of its two plurals, locī continues the PIE plural, while loca continues the PIE collective. Lewis and Short describe the difference in meaning as follows: "plur. loci, single places; loca, places connected with each other, a region" -- where the plural vs. collective meanings are clear. Locus actually isn't the only such noun in Latin: there's iocus "joke", pl. ioca, and carbasus "sail", pl. carbasa. And there's at least one case in which the collective was reinterpreted as a feminine singular: pila "ball", which seems to have originally been the collective of pilus "hair" (meaning "mass or ball of hair").
(By the way, another heterogeneous noun with a similar story behind it is frēnum "rein", pl. frēnī. In this case the Latin masculine plural ending continues what was in PIE terms not a plural at all, but a neuter dual.)
