Some of the older ILS that I flew on, gave false glide slopes at the harmonics of the GS angle i.e. 6 deg, 9 deg, and 12 deg, for a 3 deg slope. The signals were reversed at alternating angles; thus although a false path existed at 6 deg the fly up / down indications indicated in the opposite sense. Some 9 deg GS were use for experimental work on steep approaches. Some military ILS with GS as low as 2.5 deg had correspondingly lower false slopes at 5, 7.5 and 9 deg, but again with alternating reversals of the fly up / down indications.
There is nothing to say that you must not join a GS from above, but it is not good practice. Good situation awareness is always required, but with joins from above or when flying a CDA a mental plot of altitude vs range is essential. Remember 3 deg slope is approx 300 ft/nm thus you should be on the slope by 1500 ft (aal) at 5nm.
Also, remember that there are similar side lobe effects on the LOC, thus automatic / FD systems should not be armed for capture until close to the centreline, which in turn must be verified by other means. ADF locator, map, crosscut DME, etc. For me this is a far greater hazard than a false GS; early LOC capture and turn in can quickly erode safe altitude margins.