Generally, there are dual GR markers: the "go this way" and the "don't". In doubt, I walk a couple of minutes both ways; generally you find another, reassuring marker. Click
here
One problem are local routes, which are similar to the GR markers, but painted in other colours. In some Caminos, actually you have to follow local routes (as in the Bavaria, with the Königsweg); or the "Chemin de Saint Jacques" does not follow exactly the GR routes (as in the Piedmont).
As for lost or weird markers, I present you with one of my favorites, in the Munich jakobsweg: the tree with the marker of the Königsweg (a "K" with a crown) was cut, but the kind lumberjacks let the log on the ground...I missed it at first, and finished in a marsh.
I remember what an old hospitalero told me once: being lost in the Camino is part of the experience.