What happened?
I had placed construction sites for Tunnels in a room, but changed my mind because of the high energy cost.
When I deleted a construction site that was already in progress, energy dropped on the construction site location - which, given that this was a tunnel, was on the terrain wall:

https://screeps.com/a/#!/history/shard2/W41N24?t=10647462 At the linked tick one build action has just been completed. The next tick the constructionSite is removed and the energy on top of the wall at (10, 20), though barely visible because the pile is only 11 energy big.

What should have happened?
I'm not really sure if the current behavior is intended behavior.
Personally I would expect the invested energy to drop on the nearest non-wall tile, or not drop at all.

How can we reproduce this?

1. Place a tunnel/road constructionSite on top of a terrain wall.
2. Wait for some building progress to have been made.
3. Destroy the constructionSite.

If the construction sites were removed in a different order, the site on (11, 20) would have been the one that had the progress and therefore energy drop on an inaccessible location.