Error message is
Error while running event nixie-tubes::on_tick (ID 0)
invalid key to 'next'

I was building a new display - right-most digit already placed, construction bots just placing several more digits to the left. At the same time, I was connecting a red wire from a chest to the right-most digit - possibly at the same tick as a construction bot placed other digits.

All bug reports shoudl be going to github, as I'm unlikely to notice things posted here due to lack of notifications. That said, this is a bug i've been hunting for some time - it shouldn't be happening anymore, but it still does occasionally... :(