What happens?: {{#language:sr-ec|en}} returns 'Serbian (Cyrillic script)'
Taken at face value as an IETF language tag, sr-ec decodes to: Serbian language as spoken in Ecuador. I suppose that might be a reasonable interpretation but it seems to me that such a use would be rare enough to not warrant a specific tag in MediaWiki's languages lists.

Of the nine languages that include the disambiguation (Cyrillic script), all except sr-ec use the ISO 15924 script subtag -Cyrl to denote Cyrillic script (kk-Cyrl, tg-Cyrl, tt-Cyrl, uz-Cyrl, ady-Cyrl, crh-Cyrl, kbd-Cyrl, and ruq-Cyrl)
What should have happened instead?:
sr-ec should be changed to sr-Cyrl