Confusing sentence in 16.4 Anti-patterns

[fBedecarrats]

Many thanks for this wonderful book.
There is a sentence in section 16.4 on anti-patterns that I fail to comprehend. I wonder if a word is missing or misspelled:

• If the head() throws an error, the observe() will terminate the app, but reactive() will propagate it so it’s displayed reactive throws an error, it won’t get propagated.

This fragment: "so it’s displayed reactive throws an error" really puzzles me...