back to front?

[Eway]

From a grammar exercise book:

"Jim:
You put the fountatin in the middle of the room.
You had the plans back to front.
You thought they wanted the fountain in the living room, not the garden.
But don't admit anyting,
Only tell them what you didn't do!"

What does "had the plans back to front" mean?

[Marylin]

From a grammar exercise book:

"Jim:
You put the fountatin in the middle of the room.
You had the plans back to front.
You thought they wanted the fountain in the living room, not the garden.
But don't admit anyting,
Only tell them what you didn't do!"

What does "had the plans back to front" mean?

Doesn't sound natural to me. Shouldn't that be "backwards" instead?