I think I may have done this once. I paid for a more expensive service, and it was my last time getting this service. I meant to give a $20 tip, which would have been fairly generous. But as I went to leave, she ran to the door and called behind me, "Did you mean to leave this much?" and waved it. I told her yes, and went on my way feeling pleasantly generous. And then at some point later I remembered that I thought I might have had a $50 in my purse, a gift for my birthday. Had I given her the $50 instead of the $20? That haunted me for a while. But she'd certainly given any potential mistake her due diligence, so it was my fault if I had. I was never quite sure if I'd broken the $50 previously or if I'd given it to her. Oops.
I'd ask her the next time, "So, I think I've been misreading this sign the whole time... fills, are they $15, or $30?" And if she says, "$15," then laugh at yourself and say, "Man, you'd think I'd be able to read a sign correctly after all these years. Well, that will certainly make my life easier, as money has been a bit tight lately and I'd like to be able to keep getting my nails done!" and then give her the $15 plus whatever you'd usually tip on it. If, however, she says, "$30," say, "That's what I'd been thinking, but then I saw that sign and got confused," pointing to the sign, and wait for her to clarify.