Things that can go wrong

First of all, a few sad tales of what can go wrong with timing at this or any other Head Race (I have had direct experience of all of these!)

Assumption that one knows it all, and failure to read the notes. Fatal.
Non-electronic watches: Keeping bad time. Going slow or fast, or not wound up.
Electronic watches : On a cold day, all the watches being used for a race stopped because the battery voltage dropped in the cold weather.
Bad traffic caused synchronised watches not to arrive at the finish in time.
The stake boat on which the start time-keepers were placed drifted on its anchor. The new position chosen for the boat was so far from the racing boats that it was difficult to see the bow numbers.
Someone who had said that he could use the stopwatch provided, discovered after two or three boats had gone by that he did not after all know the watch well enough to use it properly.
The paperwork provided by the secretary was set out it a way that made recording of times easy on the face of it, but full of pitfalls in practice. (Not HORR!)
Radio, mobile phone and direct computer links fail.
Timekeepers lose concentration, and become incompetent.
The sun gets in your eyes.
Paperwork drenched in water.
Hands too cold - writing impossible.
Boats lose their numbers.
13 boats come by the line within 3 seconds.
The timing team fails to cooperate, and becomes inefficient.
Data lost after transfer to computer.
Data on two computer systems incompatible.
Wrong button pressed stops the watch instead of taking lap time. Can be done by experienced timekeepers at random.
Conditions so cold/wet/windy - timing difficult/impossible
Timekeeper unable to see finish line.
Timing disrupted by onlookers.
Computer program failed.
Timekeepers did not turn up for duty.
Working conditions difficult - insufficient room, chairs, tables, etc.
Tape recorder or video evidence camera fails.
Work (particularly post-race calculations) disrupted by competitors wanting results before they are ready.

As well as some of the general problems outlined above, the following have caused difficulties in the past:

Timekeepers need to be aware that the race goes on for two hours. It is very easy for concentration to lapse, particularly after the first hour or so.
Conditions are cramped at some timing points. Several people have to be on the timing line, and other members of the team have to be fairly close to them.
Crews can arrive in bunches, which require dealing with quickly and confidently. We have had insufficient practice in the past.
A large number of people is needed at each time point, because we aim to take three independent records of the time. If only two times are taken, and one is wrong, there is no means of telling which of the two times is more accurate.
A large amount of information has to be passed between various members of each team. Information overload can be a problem, and everyone has to take care not to cause meltdown.