I get this sometimes when placing a Rail Loader:

Error while running event railloader2::on_robot_built_entity (ID 17)
railloader2/control.lua:139: attempt to index local 'rail' (a nil value)
stack traceback:
railloader2/control.lua:139: in function 'create_entities'
railloader2/control.lua:203: in function <railloader2/control.lua:196>
(...tail calls...)

I get something similar:

The mod Bulk Rail Loader 2.0 (2.0.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event railloader2::on_robot_built_entity (ID 17)
LuaEntity doesn't contain key circuit_connection_definitions.
stack traceback:
[C]: in function 'index'
__railloader2/ghostconnections.lua:21: in function 'get_connections'
railloader2/control.lua:158: in function 'create_entities'
railloader2/control.lua:203: in function <railloader2/control.lua:196>
(...tail calls...)