The Event Standards Committee has considered two appeals in the past year following disqualification of competitors whose punch at a control failed to register electronically where SI punching was being used.   In both cases the appeals were made on the grounds that the competitors had been seen to punch at the control and,  in both cases, it was clear that the control unit was functioning properly both before and after the competitor had punched.   Both appeals were turned down and the disqualifications were upheld.
So competitors at events using electronic punching are advised to take care when they punch.  If the SI system is being used, the control unit will bleep and the light on the control unit will flash.  If the competitor's punch produces either of these signals, the punch will have registered on their e-card/dibber.   If you are not absolutely sure that you have heard the bleep or seen the flash, go back and punch again as Jon Duncan did at his last control for the relay at the recent World Champs. 
Particular care is needed at busy or noisy controls.  Where there are several competitors at a control with a number of punching units, the bleep you hear may be from another competitor's punch.  If there is background noise, such as the last control where there may be spectators cheering you on, you might not hear the bleep at all.  In these circumstances, it is best to look for the light, rather than simply relying on the bleep.
The responsibility to punch correctly is firmly the competitor's.  The relevant part in the rules follows in italics:
6.5.2  "The control card, electronic or otherwise, must clearly show that all controls have been visited. A competitor with a control punch missing or unidentifiable shall be disqualified unless it can be established with certainty that the punch missing or unidentifiable is not the competitor's fault  and that the competitor visited the control....
Those of you who have bought the new faster SI-card should be aware that even with that it is possible to punch too fast for the card to record a punch.  It is not worth trying to gain a fraction of a second and risk disqualification.
The first and fastest part of the SI punching process is when the SI unit reads the card.  The longest time is taken to write the data into the card.  So even if you punch too fast and don't get the data written to your card, the unit will almost certainly have read the card number and recorded your visit.  However, it is not allowed for that data to be used to reinstate the competitor; if it were, we'd never get the results out until all units had been retrieved from the forest and interrogated.  The new IOF rules make this completely explicit with the following wording: '20.7 ...If a competitor punches too fast and fails to receive the feedback signals, the card will not contain the punch and the competitor must be disqualified (even though the control unit may have recorded the competitor's card number)'.

Event Standards Committee  (from Ian MacMillan,  YHOA Rep)   January 2004