I'm not sure of NB23's assertion that developer stains are the culprit. Even an acid stop bath doesn't wash out the residual developer. Usually, the time in an acid stop is only 30 seconds or so, not nearly long enough to remove the developer from the emulsion. Some of it gets carried over to the fixer anyway, no matter what.
Best,
Doremus
Yes some developer goes into the fixer but with modified alcalinity.
I have experienced all this, first hand, especially as a teenager where I sometimes did not even bother to use a bath, any kind of bath, between developer and fixer. Stains were always the result.
And emulsion being an emulsion, film or paper, the test is easy to make.
It costs 1$ and takes 2 minutes.
Take a 5x7 print preferably Fiber base, and develop it. While it’s in the developing bath abd somewhere half way into the development stage start pouring fixer into the developer bath. This has now become a bath where fixer and developer are battling.
Watch the reaction; one way the paper is getting developed and at the same time it is fixing, and brown stains start to form. While the paper is wet the look is quite nice, but once it dries down it becomes ugly to look at. This is a nice way of creating photos with a ratty vintage look.