I think that when spelling is as standardized as it is in English, you can reasonably regard the use of archaic or obsolete forms as an error or mistake. Of course, I think it's important to keep that in perspective. Saying that you "shall be haunted by that perversity" is a severe overreaction; saying "this form is not commonly in use and will be negatively regarded by others" is entirely appropriate.
I would assume that the author of the sign chose that spelling because that's how the words sounds and she didn't know the standard spelling, not because she was relying on established historical spelling.