when the grounds break
When I used PHP, MySQL, Apache and CakePHP, then something broke, I wouldn't know if it was PHP, MySQL, Apache or CakePHP that broke.
The source code of big projects is huge like nobody knows the whole code. Do you have to know PHP, MySQL, Apache or CakePHP's source code? CakePHP probably has the smallest source code, but even so it's big, because it's a full-stack framework. Do you have to know CakePHP's source code?
When something breaks and you look at the stack trace, you see lots of return statements. If you don't know the whole source code and how the functions interact, how come you know what went wrong?