I typed a comment for a questions, and then decided to make it an answer when it grew longer and more detailed than appropriate for a comment.
I copied and pasted my comment into the submission form, extended it, and pressed Submit button. The spoiler CAPTCHA came up, which mystified me for a couple of seconds. Then I accidentally dismissed the CAPTCHA (not quite sure where I clicked to do so) without answering it, and the question opened in a new window with my answer present.
This is a spoiler