I seen the 1966 sms dime with 5 on cheek on ebay. This 1999 D dime has a 5 on the forehead. I included an example of the 1966 sms dime. Since there is no reason for a 5 to be on this coin, it must be pareidolia, right?
Correct on Pareidolia.. Just like the 5 on Cheek issue. If I remember correctly the 5 on Cheek was caused by a piece of lint on the Die. It is not an actual number 5. Just appears to look like it.
@Avery G. Mr. Weinberg posted an explanation of "5" in the check in a post about a year ago. You can look it up in the archives. As @paddyman98 said, it's just die damage that resembles a "5" if you look at it hard enough. Some people seem to be hyping it as something special, but experienced collectors know it's not significant. As for your coin? You provided the answer in the last line of your OP
And to think I gave you some credit when I first came back, Do you think you really have a five in the fore head dime? Your coin may be suffering from some indirect design transfer. Seems a little more reasonable than the dropped letter thought. these can happen in Whitman folders.
Congratulations. You finally got something right. Let's hope this starts a positive pattern as it is in fact coin pareidolia.