Thank you all for the replies. Indeed I had mixed the whole bottle to working solution (which I agree was probably a mistake). I'll bin it, but could someone first mention why such a tiny amount of fix should ruin the whole bottle, please?
Some developer/fixer mixes can work together as monobaths, but they work in a substantially different manner then when you use the chemicals separately.
And I believe that monobaths need to be mixed up just before use.
When you develop your film, you are counting on certain temp, certain agitation, and for certain time will give you a consistent result. You LOST that.
Also, consider what fix will do to your film.
Developer will turn exposed part of the film into silver
Fixer will remove what wasn't developed into silver and down the drain (sort of speak)
If you have fixer come into contact with film before developing your film, it will start to remove all material. Having fixer IN developer will cause (in theory) start removing emulsion material from the point you pour your now contaminated developer into the canister - exposed or not. Remember, once gone, it's gone permanently. Most of us won't risk it.
Generally speaking, contamination in forward direction (the order you do your processing) is not that much of a problem. Backward is a pretty big issue.
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...or you could just mix up new developer and label your chemistry, if you really prefer the old-fashioned approach.
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With about 8% of your developer being fixer, the developer may have some activity left in it, but I'm pretty sure it will be quite weak and certainly not to specification - even if the dev still worked at all, you wouldn't know how long to develop for. You could test with a short piece of scrap film with some meaningless test images but certainly don't use it on anything you care about.
Let's share how we avoid such mistakes then....
Like the guy above me and skip one and the one above that (oh, wait, that's me... I'm not the chosen one either), I've made such mistakes and ruined a whole bottle of my chemistry before. Since then, I use a system to stop myself from doing it again.
My rule is simple. NEVER have more than one bottle open at any given time. So as I'm done with a step, I pour the chemical from the tank to a cup, then it goes into the open bottle. Pretty simple but effective. Also, as I line up my bottles in the order I need to use it, and place a measuring cup right in front of it. So there is never a confusion what's in each cup. Chems travel vertically from a cup to a bottle.
It's pretty easy to make careless mistakes. I try to avoid it by doing the above and making it a habit.
Fixer is the mortal enemy of undeveloped film. It dissolves it. That's its job.