I found a workaround: don't store anything not serializable not just in global but anywhere it can be accessed by metatables (from objects in global).

mod page https://mods.factorio.com/mod/SandboxedLuaCombinator
It adds combinators programmable in lua.
I am developing mod, and at some point it become impossible to save game when any of my mod's combinators are active, while I can't find that I placing anything wrong to global table. Moreover ...

I recently found mod named LuaCombinator2 , and I liked it's idea but not implementation, so I 'improved' it (if I correctly understood CC BY-SA 4.0 license it is fine). So here it is:

Sandboxed LuaCombinator
https://mods.factorio.com/mod/SandboxedLuaCombinator
Adds combinators programmed with ...

We can use select(start, end) to change selection or cursor position in text fields, but looks like there is no way to know where cursor is. I am making ingame lua code editor with syntax highlighting (programming combinators), and there is working implementation, but it have to compare previous ...