A sentence, that, although well-intentioned is actually self-contradicting.
In general it is true that generalisations can be very dangerous. There are frequently exceptions to rules and patterns, especially in a social context. Thus while, in general, it's true that, say, Japanese people are polite to Westerners in Japan, and French people are arrogant towards anyone who doesn't speak French in France, one should not tar all French and Japanese with the same brush. This is basically a motherhood statement: Don't be prejudiced against people, etc etc.
But of course, the problem is that the statement itself is self-contradicting. "All generalisations are false" is in fact a generalisation.
I've heard people use the statement without event the slightest awareness of the irony of what they are saying.