If you don't know who the Tails Doll is, allow the following site to explain everything to you.
This site. Right here. Click here.
If you don't want to go there, I'll just give you a quick summary.
If you play Sonic R, and you unlock the Tails Doll, or something close to that, then you can expect to get cursed. And die. There have been MULTIPLE accounts of this. That site has over a hundred tales of the Tails Doll's curse.
Careful, though. That site can scare. ...you.
.....and that's it. Yeah.
The Tails Doll by Sonic Team. That site by Quacker and Bowen, or something. "Biography" by DJay32.