Wow! You say KLM "always" sells mixed-fare-class tickets, so "you need to carefully check each segment in each flight you select". Thanks for the heads up! I've seen mixed class airfares come up with a special icon alerting you to this fact when booking on other airlines (AlaskaAir FF award tix come to mind). Buyer beware, indeed!
Granted, I bought these because they were cheaper than economy airfare to the same destinations, but I recall that these cheap fares I was searching did not seem to depend on the mixed class nature of the flights...however, I will look close next time. Delta not going to give me any problems in SLC when I try and recheck 3X70lbs of free baggage afforded on my KLM itinerary for this Coach segment, are they?