As both a a frequent walker and bicycler, the very worst thing is when the biker gives absolutely no warning at all. This happens quite regularly. A road bike in particular is the ultimate stealth weapon, and it "jolts" me everytime one comes whizzing by with no warning as I walk.

That biker is making some assumptions about me that may be entirely invalid. He is assuming I am going to keep going in a straight line - that I won't step to my left to avoid a pile of dog poo on the walkway, for example.

He is assuming I am a rational being, and won't turn around to go the opposite way without looking back.

I think NOT giving a warning of some sort is a sign of an incompetent bicycler, yet it happens a lot.

About 50% of the walkers to whom I give a warning say thanks or wave their hand in thanks, which gives some sort of a clue about how many times they are passed without a warning!

Aslo, I have learned to always signal ANY turn when cycling, as I have been surprised a number of times when there was a stealth bicycler directly behind me about whom I had no clue, and who may be overtaking me or just following real close.