When do derailments happen?
Derailments usually happen when your goal's countdown reaches 00h 00m 00s and you're on the wrong side of the bright red line. The goal shown below is derailing in 11h 25m... unless the user adds +28, which will make them safe for today by putting their data above the line.
Beeminder processes derailments at your goal's deadline, the end of the "Beeminder day". The default deadline is midnight in your time zone. You can change deadlines, both your default as well as the deadlines for each goal, unless your goal is linked with an autodata integration that doesn't allow for non-midnight deadlines (such as Fitbit, Toggl, etc).
You may derail on your goal immediately if the goal's data or bright red line is modified such that you are no longer on the right side of the line at the last deadline. For example, deleting all your existing datapoints will almost always instantly derail you (and possibly do so multiple times); the line showing your data is dropping, but the bright red line is not, so you'll end up way off track.
This scenario usually occurs when you're adjusting or deleting previous data entries (perhaps when trying to correct weaselled data where you should have derailed already...), when trying to clear out the goal's history to restart it (don't do that), or when an autodata integration overwrites or imports earlier data. It may also happen when you tweak your goal's custom settings.
If this happens to you unexpectedly, please contact support and we'll help get things back to normal!
Keywords: timing of derailments, instant derail, insta-derail