Some say, "If it's a really complicated and/or special trip, you should consider using a travel agent, but for simple flights within the U.S, just go ahead and book online." That's not always true.
Sometimes the best laid plans of computers, especially when it comes to automated rebookings, don't work out. A story that unscores the need for humans in the rebooking chain.
The real lesson and the point of this post, is that once again, the only absolute consistency we can expect from the airlines is inconsistency.