I was not aware this existed until @FurionHuang pointed it out and translated a Chinese forum post for me https://tieba.baidu.com/p/5484766933 The overflow was discovered and heavily verified by @Rogue-star and so big thanks to both of you for this. To try and translate the translation into an overall production view A city will normally be building something and if it gets additional production from another source (like chopping) the current item being built will get this production. If it finishes any additional overflow can be added to the next item. If additional production comes from another source when nothing is in the queue then that production is not kept for the next item. This is consistent with other areas for example Gorgo killing a unit while there is no civic being built. She does not get the culture benefit.(I tested this a while ago) If you are building a wonder and at the end of a turn someone finishes it, your wonder build is cancelled (so there is nothing in the queue) and you get back 50% in production which is then lost because nothing is in your queue. The testing showed that 50% does exist because if you have spent say 200 on a wonder and can see someone is going to finish it before you, if you switch to a campus my understanding is when that wonder finishes your currently building campus (or whatever) will get 50% production back - 100 additional production applied. If it finishes you can apply any leftover to something else. Just be aware, if there is nothing in your production queue at the start of the next turn you loose any additional production. I am going to look at this last bit because I know I can overflow warrior production between turns finishing 1 turn.