It's the unlimited shields option. But (and this is the underlying reason); Halo 3 was not graphically re-worked (re-skinned) as Halo Anniversary, Anniversary CE or Reach had been. So I found leaving this option off, clears up the problem. 😉
The current Halo 3 trainer is working with no issues for me. This relates to the trainer with a game version of 07.14.2020.