I know some members here on CT collect errors coins. Orthographic legend mistakes can make common coins becoming “special” or even “unique” examples. Would you pay more to acquire such specimen ? Maybe.... Currently for sale on Ebay is this Galba denarius very unique because of...his holder error ! The reverse legend clearly read ROMA VICTRIX but on the slab it’s described as VIRTVS. Probably a Friday afternoon blunder... But tell me : would you pay more for it ?
No, I wouldn't. However, I like the coin and that is the reason I would buy it provided the price is reasonable of course
If it is a mint error, of course! I find them interesting. But someone's mistake on a slab, no way! That isn't what I would call an 'error' coin. When I purchase a slabbed coin it is promptly removed from the tomb in any case. Here is a true error coin I acquired earlier this year. The error is pretty easy to spot too. And no slab is required!
I would not pay extra for the "error" slab. However, as a collector of Galba denarii that is a great coin worth serious attention.