This occurs specifically with the Factorissimo mod (https://mods.factorio.com/mod/factorissimo-2-notnotmelon) when a loader from this mod is set up to lead into a factory connection.

The mod Loaders modernized (1.0.5) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event loaders-modernized::on_built_entity (ID 6)
Entity is not loader.
stack traceback:
[C]: in function 'index'
__loaders-modernized/scripts/loaders-modernized.lua:28: in function 'snap_to_belt'
loaders-modernized/scripts/loaders-modernized.lua:112: in function 'snap'
loaders-modernized/scripts/loaders-modernized.lua:133: in function 'handler'
core/lualib/event_handler.lua:72: in function <core/lualib/event_handler.lua:70>