I recently selected a meal for a domestic flight, got the "failed to save" message, logged out and back in, and saw that the selection was in place.

I also recently received a similar message while changing a seat assignment. Logged out and back in; new seat was already assigned.

I sometimes wonder if Delta's systems have developed the digital analog (I know that's a contradiction) to a psychosis based on so many past failures that it simply assumes it won't be able to accomplish the requested task and is just as surprised as we are when it actually does.