Although it is said one should never mix iron gall ink with dye based inks, I know of a case or two where the mixtures did no harm, were not disasterous, and resulted in interesting colors. Me, I still stay away from that combo just for my own peace of mind. Like no swimming until an hour after eating.
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"My two fingers on a typewriter have never connected with my brain. My hand on a pen does. A fountain pen, of course. Ball-point pens are only good for filling out forms on a plane." - Graham Greene
I also have a bottle for remnants of inks that I've flushed when I'm preparing to refill the pen or before cleaning a pen. Even if I'm using the same color of ink to refill, I try not to have old ink drip into my fresh ink. The resulting color is a shade of black and waterproof. I haven't had any globs in the 3 years that I've been doing this.
I have had sac in my Esterbrook desk pen (which uses the "flushed" inks) and the sac in my Waterman 452 1/2V (which has used a variety of different inks but I've flushed it clean before changing colors) turn into a gummy mess.
I don't know if the mixed inks have degraded the sac in the Esterbrook or if the sacs were defective. I've replaced them with new silicone sacs.
That reminds me...I need to buy more replacement sacs.