In Multiplayer for some reason when using the Upgrade Planner a lot of the time the items of the previous tier land on the ground. That can clog your Factory with random Belts in Item Form on resource lines.

Is there a Chance you check out Multiplayer Bugs?

This doesn't sound like something my mod can do. My mod destroys buildings and then adds items to the storage, it can't destroy a building and put item on the ground. Upgrading works like constructing and deconstructing at the same time.

Can you test if this happens with no other mods enabled and/or in single player? Send a save and/or a mod list as well, I'll check it.

This is my Modlist. No Savefile needed, this happens in any save. Just Build like a couple hundred yellow belts then Plan their upgrades to red and let the Quantum Fabricator mod Upgrade all of them From stored Items.

The Behaviour is the same as if destroying a full chest with a too full inventory, items just overflow and land on the ground.

(This only happened to me in Multiplayer)

base
elevated-rails
quality
space-age
4k_planets
aai-loaders
cargo-bay-inserters
change-recipe-quality-without-re-selecting-recipe
cheaper_aai_loaders
DiscoScience
EasyQuality
even-distribution
faster-hand-crafting-speed
flib
ic-more-qualities
InstantRobotChargingUpdated
levelup-forked
loader-lane-filter-stacks
quality-miners
quality_unlocked
quantum-fabricator
rainbow-character
RainbowCollector
RainbowLasers
remove-productivity-cap
Science_pack_glow
some-gleba-science
too-many-prods
visible-planets

Well well, I'm stupid.

In fact, my mod behaves NOT as I though it would behave. I was convinced upgrading is just construction+deconstruction at the same time, but it reality when upgrading with my mod the game would always put the items in player inventory and spill them on the ground if the inventory was full.

The bug should be fixed.

That explains it! Thanks for fixing.

That also explains why it only happened in Multiplayer for me, I usually keep my Inventory clean but my Mate doesn't xD