This happened when a bot tried to place a loader, using force-place or whatever the angry ghost is called.

The mod Bulk Rail Loader 2.0 Temporary Patch (2.0.4) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event railloader2-patch::on_built_entity (ID 6)
Unknown entity name: speech-bubble
stack traceback:
[C]: in function 'create_entity'
railloader2-patch/control.lua:33: in function 'show_error'
railloader2-patch/control.lua:207: in function <railloader2-patch/control.lua:204>
(...tail calls...)