Coin Weight Tolerance

The spec weight for a '99 dime is 2.268 gm, and it has a tolerance range of +/- 0.091 gm meaning coins between 2.359 gm and 2.177 gm are within mint tolerance levels.

That means your coin is between 0.419 (the max) and 0.237 (the minimum) gm under specified weight. That's a lot for a dime !

That said, without close examination by someone who knows what they're doing it's hard to say if it's an error or not. But even it were an error it would probably only be worth a few dollars at best - and I mean few in the literal sense. As a general rule most error coins have very little monetary value. They are collected mainly as oddities by people who like such things.

To put that into perspective, if it were an error, it would probably cost you 2 or 3 times its dollar value just to have it slabbed and certified as an error. So would it be worth it ? Not likely.