Nice catch :), except that I accounted for that above, that's why I quoted the parent's single line, to mean that the substitution applied only to it.

But it would have been more clear if I had written it thusly:

echo "I wonder about photography." | sed 's/graphy/shop'

Also, if the expression had been wrong, other words too could cause it to fail, like videography, filmography, orthography, etc.