invalid byte sequence for encoding "UTF8": 0xf481 - how could this happen?

My db is in utf-8, I have a row in my table say tmp_article and I wanted
to generate ts_vector from the article content:
select to_tsvector(content) from tmp_article;
But I got this error:
ERROR:  invalid byte sequence for encoding "UTF8": 0xf481

I am wondering how this could happen. I think if there was invalid UTF8
bytes in the content, it shouldn't have been able to inserted into the
tmp_article table as I sometimes see similar errors when inserting
records to tmp_article. Am I right?