because there was a broken plank which the filly fell through.

I would just like to point some things out about this image:
I made the first part myself
I was not, however, the one who added the Derpy panel
I only uploaded the original image to only two places – KYM and My Little Brony.
The My Little Brony upload had the blue bar added to the bottom
The bar is missing from the bottom of panel 4
Which means that someone from KYM found the image, added Derpy, and uploaded it to MLB where it made the front page.

So my question is, who was it? You deserve a muffin!