Steps:
1. Select signal 1 (Iron plate etc)
2. Select second signal slot without entering values or pressing 'E'
3. Select signal 2 (Copper plate etc) in new slot
4. Press 'E' without entering values
5. Copper plate wont show in list.
6. Selecting Iron plate will crash

While in the bugged state from step 3, two crashes are possible.

1. Selecting the original signal:
   4333.004 Error MainLoop.cpp:1432: Exception at tick 3616371: The mod Cybersyn Combinator (2.4.0) caused a non-recoverable error.
   Please report this error to the mod author.

Error while running event cybersyn-combinator::on_gui_click (ID 1)
LuaGuiElement API call when LuaGuiElement was invalid.
stack traceback:
[C]: in function 'newindex'
__cybersyn-combinator/scripts/gui.lua:620: in function 'handler'
flib/gui.lua:177: in function <flib/gui.lua:160>

1. Selecting another signal slot
   4689.836 Error MainLoop.cpp:1432: Exception at tick 3603460: The mod Cybersyn Combinator (2.4.0) caused a non-recoverable error.
   Please report this error to the mod author.

Error while running event cybersyn-combinator::cybersyn-combinator-confirm-gui (ID 208)
LuaGuiElement API call when LuaGuiElement was invalid.
stack traceback:
[C]: in function 'index'
__cybersyn-combinator/scripts/gui.lua:685: in function 'set_new_signal_value'
cybersyn-combinator/scripts/gui.lua:744: in function 'confirm_signal_value'
cybersyn-combinator/scripts/gui.lua:3204: in function 'on_input_confirm'
cybersyn-combinator/scripts/gui.lua:3287: in function <cybersyn-combinator/scripts/gui.lua:3287>

Thanks for the report! There was a slight issue with the "E to confirm" feature where it wouldn't properly reset state when activating a new signal slot while a previous one was active.

v2.4.1 is out with a fix for this.