What's wrong? "The only thing growing in abundance was chaparral and weeds."
Another kind of agreement problem
This was tricky. Congrats to 13% of you.
It isn't a subject-verb agreement problem because the subject is "thing" and the verb is "was." Those agree.
It can't be spelling because everything is spelled right.
The other choice is another kind of agreement problem. It is true that "chaparral and weeds" is plural whereas the subject is singular, but it isn't incorrect, tough I wouldn't recommend writing this sentence because it does seem awkward.
I would rewrite it in one of two ways:
1) Make the subject plural: "The only things growing in abundance were chaparral and weeds."
2) Recast the sentence: "Chaparral and weeds were the only things growing in abundance."