ADF and Doppler
Scientists have recently found an answer to the problem of station passage over an NDB, which is often part of a question in commercial pilots' exams. Many people have been confused, when working with a VOR as well, on finding that their position has not actually been over the NDB at all.
The reason has been found to be Doppler effect, in that, when going towards or away from the beacon, the radio frequency shifts slightly, up and down, respectively, so the ADF equipment is never really tuned properly, and you are therefore never really over the beacon.
The only aircraft known to have successfully done this are those with the old "coffee grinder" tuning method, which can thus tune upwards slightly on the way to the beacon and detune after passage.
Phil