Words that appear plural but aren't:
Species. Many scientists have started referring to a "specie", in an incorrect attempt to avoid using the plural. But "species" is a Latin 5th-declension noun. In the 5th declension, both the singular and plural end in -es.

Words that appear singular but aren't:
1. Apparatus. Some scientists find "apparatuses" too awkward, so they try to form the plural as "apparati". But there are two types of Latin nouns ending in -us. Second declension nouns form the plural by changing -us to -i. But for fourth declension nouns the singular and plural are the same -- both end in -us. Generally -us nouns that originate as participles ("apparatus" means "prepared") are fourth declension nouns. So according to Latin rules, the plural of "apparatus" is "apparatus".

2. Data (plural of datum). The data ARE (not is) consistent....

3. Lots of Italian words related to food:
zucchini (singular zucchina)
broccoli (singular broccolo)
raviolo
spaghetto
etc etc