MCPA alters the metabolism of ryegrass and wild oats in such a way that these grasses no longer respond to diclofop, even though MCPA is not itself toxic to them. The chemicals must be applied at least 10 days apart. Bromoxynil reduces this antagonism. A mixture of these three herbicides can be used successfully, and indeed is available as a commercial product. A tankmix of MCPA and diclofop will still control wild radish.
Some herbicides increase the activity of others.
Most emulsifiable herbicides increase the activity of flowables by acting as a spraying oil. You cannot determine biological incompatibilities by doing test mixings. The herbicides may be perfectly stable in the tank. It is only after the tankmix is applied that the problem becomes apparent. Weeds may not die if there is antagonism, or the crop may be badly affected if there is detrimental synergism.