From The Mythical Man-Month, 20th Anniversary Edition (1995), a seminal and timeless book of essays on programming by Frederick P. Brooks, Jr.

[One of the woes of programming] is that
designing grand concepts is fun;
finding nitty little bugs is just work.
With any creative activity come
dreary hours of tedious, painstaking labor,
and programming is no exception....
So testing drags on and on,
the last difficult bugs taking more time to find than the first.