If you are an avid user of Forge, I recommend you stay away from the Halo 3 iteration if you wish to make serious maps. This is due to an issue where the game does not accurately save the float value we use to edit the objects in Forge. This results in tilted and shifted objects in the map.
Here is a visual demonstration of the issue. You will have to copy and paste the link into your browser.
https://www.youtube.com/watch?v=thEWQNgHXqY&feature=emb_title
Here is a more technical explanation by someone not from 343 who was investigating this issue with modding tools:
"The half-float data type is inspired by the IEEE 754 standard, except sacrifices range and accuracy in favor of representation size. A half-float comprises a sign bit, a 5-bit exponent with a bias of 15, and a10-bit mantissa
This is why Halo 3 map objects never save in place. Range and accuracy of the float is sacrificed for a smaller files size. Instead of saving the float to a 4byte value they use HalfByte precision to save it as 5 bits (2 and a half bytes). Don’t think there is anything i can do to make the floats rounded to 0 so we’re stuck with inaccurate object placement on Halo 3."
343 would have to change how the engine saves .mvar files to fix this issue, which I do not know the difficulty of but I’m sure it must be hard. If it can be fixed, it will be awhile before it is.