It happens all the time!
Here's a recent example from the November Gen Nav JAA exam:-
"In which of the following situations is the FMC present position of a B737-400 Electronic Flight Information System likely to be least accurate?
a Top of climb
b On final approach
c Top of descent
d Just after take-off"
The only possible answer to the question as printed is "It depends. What is the state of your DME/DME cover and are you using the ILS for the final approach?"
On take-off, you'll have operated the TO/GA switch as you advance the throttles for take-off, so you'll have just updated the FMC to the position of the runway threshold held in the Nav data base, so the answer is not (d).
I think that the JAA expect you to have had good DME/DME cover over the cruise, giving the Kalman filter lots of time to generate a good accuracy at top of descent. I would expect 200 metres or better. But only if you've had good DME cover. Top of descent might be bad, if you haven't.
I think that the answer they expect is (a) - top of climb, because you will not have had good DME cover for very long.
I think that on the final approach, they expect that you would be using the ILS, and the localiser is used to update the FMC position virtually without passing through the Kalman filter. But there may not be an ILS where you're going to. Also, if there is no co-located DME to correct in range as well as azimuth, it's only going to be corrected in one dimension.
Furthermore, if there is good DME/DME cover during the climb, but the destination is in some unsophisticated airspace environment, there may be poor DME cover at top of descent, making that worse.
Basically, it's a lousy question, which can't be answered without more information. (a), (b), or (c) are all possible answers, depending on the conditions. There are plenty of examples of other equally poor questions.
There is a procedure in which the CGI of your FTO can pass comments about exam questions to the CAA and they will either give credit if the question is unreasonable, or delete the question for future exams. As an individual candidate, you can do very little. But you can pass your comments through your school.