I have discovered what might be a bug within the keyboard mapper. To reproduce this, use the following steps.
1. Map the pause key to the play/pause media key.
2. Launch a game that uses the pause key for pausing (such as Ubi Soft's Rayman).
3. Pause the game.
4. Unpause the game.
5. Close DOSBox (Control-F9).
6. Reopen DOSBox.
7. Launch the game again.
8. Press the play/pause media key. Nothing should happen.