Welcome to the community. Trust Photo Engineer's views for chemistry stuff - He had a long career doing that stuff professionally, and has been a wonderful asset in the film community.
For added clarity, it might be worth noting which specific developer you were mixing up.
This thread has me wondering if any photo chemical kits use an in-order reaction process to create chemicals at time of mixing, or if they're all more or less divided up to avoid early reactions and for ease of mixing.
Ex. Mixing Part A with Part B creates chemical X, which is transformed into Chemical Y after adding Part C sort of thing.
Personally I find that careful preparation helps to avoid mixups like this, and reinforces my confidence in my process. - If I'm mixing chemistry, I'll prep all the components and lay them out in the order I'll need them with the first item nearest me in the line. This gives me time to double check that I have everything I need and in the order it is supposed to go in before I unseal anything.
This means that if I'm reaching over something for my next step, then I'm obviously about to do something foolish and should probably stop and rethink. [My professional background has been in software QA and User Experience, so my typical mindset is that 'if something can go wrong, it probably will, and odds are the user will eventually be as stupid as possible, so give them as few options to be stupid in the first place. And I try not to exempt myself from that pool of users.]
So far it has prevented me from fixing my film before developing it, but I'm going to assume I'll eventually manage that one.
