Gif of this in action: https://i.gyazo.com/e45bb8bc3e62f824e42983e75543424e.mp4

The filler will attempt to place saplings on top of stone. It will also try to place them on air blocks. When this happens it will keep trying, getting stuck in a loop.

This loop causes the filler to freeze completely, even if there are other valid places for it to place the sapling.

When it gets stuck in this state, any blocks it's trying to place will be lost. They get stuck in transit forever, and they aren't in the inventory. Breaking the filler voids these blocks. This one might be a separate bug.

BuildCraft version: 7.99.22
Forge version: 14.23.5.2815
Link to crash report or log: https://gist.github.com/TheDeviantCrafter/59a7329f77900ea77d4457908fe90f76
Singleplayer or multiplayer: Singeplayer
Steps to reproduce:

1. Create a square of stone, or another block that saplings can't be placed on

2. Place land marks and a filler so the filler will try to place blocks on those blocks

3. Place some saplings in the inventory.

4. Set the filler to fill mode

5. Give the filler power

Additional information:

The bug will also trigger if placing torches on air or tall grass on stone. Both are cases where you couldn't place the block in vanilla.