Thanks for the nice mod at the first place.

Recently I have run into error - steps to reproduce:
1) Using ~30 mods, SE most important of them. No problems so far however after 100+ hours playing
2) Set sign to display value from circuit signal
a) no wire was connected
b) text started with |#| mark - so far it properly displayed "0 text..."
3) Switch under Choose an input to print - Enemy evolution (did this just out of curiosity) and ... thud, got error message:

The mod Holographic Signs (0.0.8) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event holographic_signs::on_nth_tick(300)
holographic_signs/control.lua:413: invalid use of '%' in replacement string
stack traceback:
[C]: in function 'gsub'
holographic_signs/control.lua:413: in function 'apply_holo_sign'
holographic_signs/control.lua:508: in function <holographic_signs/control.lua:504>

Played a bit around yet and seems, that you must not (if not willing to get crash ;) ) place |#| for some input sources. Have not tested thoroughly, but at least evolution and rocketes launched are the example. Removing |#| removed crashing.
Seems you hardcode the text from said inputs AFTER the players text, independently on |#| mark position. Probably you process the text replacing |#| placeholder differently in case of circuit signal an differently for other sources...

Fixed. Thanks for the report