This proverb is an expression of a pessimistic view of life. If something goes wrong, the worst possible outcome often will probably happen.
Apparently, there is a “Buttered Toast Phenomenon” and there are some attempts at explaining the proverb scientifically.
If you had the time and wherewithal to test this phenomenon, how many times would repeat the test before you tried a different sandwich spread?
Does bread always fall on the Marmite side?
How about the Vegemite side down under?