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.