At many airports in eastern Europe and in Asia, there is a switching system in the NDBs for the runway approach. See this example for the Airport of Hrodna, Belarus:

Here you have 2 NDBs south of the runway, W and WF, on 920 kHz and 452 kHz respectively, and on the same frequencies you have G and GP north of the runway. Normally, only one pair is in operation, either the southern or the northern. This indicates the direction to use when landing.

In the case confirmed to Patrick Robic, he obviously caught both G and W on 920 kHz during the testing procedure on start-up in the morning, lasting only a short time.

(Thanks to Michael Oexner for placing the pins representing the NDBs (European NDB Handbook, digital edition) into Mama Google's earth chart)