This was discussed in last week's Animation & Rigging module meeting. A summary of the responses:

• Jason: what happens with fixed curves? Do they disappear immediately?
• Paolo: "Cycle error" might need a better term, to avoid confusion with "cycles".
• Orestis: Unclear how the last key is used -- wouldn't it be better to enable a "this is looping" setting, and avoid playing back the last frame.
• Daniel S: Idea is good, but UI design needs some attention.
• Brad: The "looping" concept could be even bigger than just FCurves and Actions, for example automatically finding the best looping point in an animation and just making it loop. Less "reporting errors" and more "helping in making it loop".
• Paolo: warning about baked simulation on top of bones, as it can cause apparent mis-matches that still result in a good loop.
• Brad: there are many ways in which to loop, should be clearer which kind of "errors" are shown here.
• Daniel S & Bassam: there are many different reasons for "cycle errors", should be clearer what the issue is.
• Bassam: The current UI is for real errors like "this FCurve doesn't do anything". These mismatches are more "warning" than error, and should also explain more about what is wrong with it.

From this, I think there are a few things to work on:

• A "bigger picture" design that discusses how warnings about specific FCurves are handled. This kind of thing should be discussed in a larger scope than just this particular use case.
• Some screenshots of what those cycle errors look like in practice, and maybe a video that shows how they show up, and how they disappear after they've been tackled.