Flag copulae/verbs agreeing with complement/object instead of subject

[hippietrail]

I've been noticing this in spoken English a lot on YouTube but I don't think it's new and I don't think it's limited to native or non-native English speakers:

Verbs, including auxiliary verbs and copulae (am, are, is, was, were, etc.) agree with the subject, so normally the nomonal phrase to its left. But under some circumstances some people get confused and make it agree with the complement of the copula or object of the verb:

• The main difference are the metadata fields.

  → The main difference is the metadata fields.
  → The main differences are the metadata fields.

[hippietrail]

(Trying to submit a PR to fix this where I found it in the docs but damned if git will play along.)