The only similar cases I’ve come across are related to input devices that have some sort of input duration component (e.g. string input, microphone input) when combined with a time limit on an event. The input device marks internally that there was a response, but it does not report to SuperLab that there was a response until certain conditions have been met. It’s possible that the event may end before those conditions are met, causing a situation where a response has occurred, but the time limit expired before it was registered.
Here’s an example:
suppose you are looking for microphone input, but the activity delay is 250ms. You are looking for this input on an event that is set to end after 1000ms. At 800ms, the participant begins responding. 200ms later, the event ends. Since this is less than 250ms, it was never registered as a response.
The same thing can easily happen with string input if you have a time limit on your event.