Putting seed from several packets into the same seed-starting containers gets me in trouble, every year. Recently, I lined up some tomato seed, using only my "standard" that my seed had better not be over 5 years old and commercial seed could be that old but special positioning would be all that was necessary. In other words, older seeds should be on the outside of the container so that faster germinating seedlings wouldn't be so likely to shade them.
I had some Sun Sugar seed from 2014. Okay, I knew that if the packet says "for 2014," that it is 6 year old seed grown in 2013. What happens? The row of Sungold seedlings emerging about 7 days earlier and nearly smother the Sun Sugar seedlings.
Oh, I think 95 - 100% show up but they are just tiny, tiny things that I have to be soooo careful not to shake up too much as I move the robust Sungolds out to cell packs. I doubt if I did it to the nth degree needed ... but I tried! Still have a few of the 2014 seed. Keep it until 2020? It would require some bother - maybe I'll close my eyes and toss it.
I had some mixed cherry tomato seeds that I didn't want to put in the garden, so I just tossed them in the empty chicken yard. They grew into huge tangled masses of plants that actually produced few tomatoes. and of inferior quality. Too bad I didn't have any chickens in there to clean it up.