Checking for Extra Impulses

Sorry Pete, I can't answer that question, nor the flip side, if an impulse is missed. I'd be more worried about that, although it would make the course longer, only in that in the dark distant past I contructed a makeshift electronic measuring device using a magnetic reed switch and a simple hand calculator, and above certain speeds the switch couldn't open and close fast enough to react. I would doubt this is a problem with the current generation of cyclocimputers, since they must be designed to measure accurately at the high speeds reached by competitive cyclists - mustn't they?

Pete:
I have just noticed your question concerning the generation of spurious impulses with the Protégé revolution counter using the single-magnet technique, the answer to which has already been given in my report at http://home.earthlink.net/~caverhall/newrevcounter/abstractcontents.htm.
Of course with one magnet you are statistical four times less likely to develop spurious impulses than with four magnets. The downside is that you cannot detect such impulses by checking the synchronization of the meter with the rim because the meter is always synchronized. However, spurious impulses can be detected by monitoring the meter. If the wheel is rocked very slightly back and forward over the zero point on the rim, the meter will immediately increment by one. If the rock is slightly greater, the meter will increment going back and going forward.
The scenario you envisage though should never happen if the simple operating instructions I describe are followed. On slowing to a stop, the measurer should monitor the meter and stop the bicycle immediately after an increment or while monitoring the rim reading. The wheel should then be walked backward or forward to the final position. In the event of an uncontrolled emergency stop the measurer should check the rim reading before releasing the brake.
Finally, it is possible to imagine a measurer that is incapable of following simple instructions and is so oblivious to what he is doing that he generates spurious impulses on a run. However this run would produce a short course and is likely to be rejected in the final analysis on the assumption that statistically he is unlikely to repeat a stop on zero in another run.

I guess the answer to the question is "no." An extra or missing impulse can't be checked for when using the one-magnet method.

This tends to make me avoid it, as I make mistakes all the time. With checking I am able to correct them. Without checking, I have no way to know whether I have a problem

Pete:
I thought I had made my case that some checking is possible in the single-magnet method! Note that the Jones meter very occasionally skips counts, but there is no self-check for this.
To generate spurious impulses with the Protégé would take a very incompetent measurer and a very unlucky one at that. The chances that he would repeat the error on the repeat ride is practically zero. I do not think that you would ever experience the problem. Since the cost in time and money to try out the Protégé is trivial, I think you ought to at least try it before making up your mind to never use. Run it alongside your Jones as a safety blanket!