If someone eats a mixture of meat and milk, does he need to wait before eating more meat or milk?
It depends if the meat in the meat/milk mixture is kosher or not
If the meat is from a non-kosher animal then one can eat milk afterwards. Many poskim hold that non-kosher meat is not subject to the meat/milk prohibition. See for instance here on MY quoting the Shach, Taz and Baer Hetev
If the meat is from a nveila or treifa (forbidden meat from a kosher animal), there is a dispute between the Rashba (who forbids it) and the Rambam (who permits it). Since this is a Torah prohibition one should be strict. See here and here for details (this has practical relevance to know what one can feed once pets) -- thanks to @kouty for pointing out this important distinction
If the meat is kosher then one has to wait the full period before eating more milk because one is "meaty", see R Binyomin Forst here (p. 131) who writes
One who, in error, ate even a large amount of dairy before the end of the six-hour period has not changed his status and must wait the remainder of the six-hour period before eating any additional dairy food. There is no basis for the common misconception that once one eats dairy, he is no longer affected by the meat meal
I heard two reasons from two rabbanim I asked
- food has no "fourth status" beyond bassari, halavi and parve, i.e., a mixture of milk/meat is both bassari and halavi
- there might be traces of meat left in the mouth which is one of the reasons one waits between meat and milk.
See also R Ovadia Yosef in Halachos Olam cited here which also says the Kaf Hachaim allows more milk for Ashkenazim but I couldn't find another posek that rules like him.
One can eat more meat after rinsing one's mouth and hands according to many authorities (but not Chabad, and unless one ate meat and hard cheese.)
As always CYLOR if you need a practical ruling in a specific situation.