Well after that title, this should really be pretty self explanatory. One thing that is worth noting as it applies to almost every one of these things is just how amazingly convincing some things are. More than once I took pieces to people with years and even decades of field experience and a couple of times a debate ensured over whether or not it really was fossil bone.

Surface bone in the field
Extant bone – Obviously this looks a lot like fossil bone and it’s texture and weight is a give away once you pick it up, apart from the smallest fragments.
Wood – You would be amazed how many pieces of dried wood and twigs make a near perfect copy of longbone shafts.
Roots – Roots protruding on the surface, or more commonly that have died and rotted inside the ground can look just like bone fragments. It’s a real pain when you spend ten minutes carefuly brushing and digging around a bone fragment attached to a possible skeleton only to discover it branches.
Trace fossils – Lots of fossilised burrows are a different colour to the surrounding matrix and typically a much lighter colour or even white and again can look like the shafts of longbones at a distance either poking out of overhangsor lying on the surface.
Minerals – All kinds of bits of minerals poking out of rocks are white in colour and simply depending on how they are exposed can make you double take.
Salt stains – Salt accumulates on the surfaces of soem exposed rocks in large irregular white patches. Guess what some of them resemble?
Rocks – White rocks abound. See above.
Plaster – The area we were working in has been heavily explored in the past and fossils collected. Those who went before covered their fossils in protective jackets of white plaster jsut as we did. Inevitably bits fall off, and stain or dry on the surface. Unlike many of the other things here, these bis of plaster are of course often found in large numbers, spread out over quite a large area, and in or next to a layer of rock that you really would expect to find bone in. Thus they really stuck out as looking like good sites from a distance, and indded must have been originally since someone else obviously took out something there leaving the plaster behind.
Plastic etc. – Bits left by previous expeditions or local farmers can again fool you if they are exposed in the right way or have eroded into a familiar shape.
Bird waste – Another insidious one. Obviously many birds (and indeed bats) use some roosts repeatedly and thus a build up of wastes under those perches is somewhat inevitable. These leave nice streaks just on the surface of the rocks, or little piles on top of them that can look just like weathered fragments of bone.
That is about all I can think of, probably going to a different site will eleminate a whole bunch of these and instead throw up new ones. I hope my eye will improve with a bit of experience, though as I said it was conforting to my ego to see other fooled with almost every one of these at some point. If nothing else it demonstrates just how many bits of vertebrate skeletons look like wood, plastic, plaster, salt stains and more.
I remember my first Triassic expedition to the Arcadia Formation of Queensland. We were looking for temnospondyls so your search image homes in on little grey pieces with reticulate ornament. Sadly the outcrop area was host to a species of plant whose seed pods dry to the same shade of grey as the fossil bone and are covered with a reticulate network of veins. I think I found one genuine piece of bone for every 10-20 seed pods that I bent over to retrieve.
Thaks Adam, that does reduce the diminishment of my ego a little more! Found anything good in SA of late (apart from the new family member of course)?
BTW Feel free to revive the ‘state of palaeontology’ meme – I was rather disappointed with the way that died, but it won’t exactly date any time soon…
I’ve mistaken white plastic pipe for fossil bones. I’m not alone – a buried pipe was misidentified as a mammoth tusk several years ago.
Having gone out prospecting for the first time in 2 years I can SO totally agree with you. Especially the roots! I went for so many roots thinking awesome! Only to be fooled DOH
I did find an Albertosaur tooth though (if only I wasn’t in a Provincal Park… so I had to leave it)
Oh man.
Reminds me of a time that we found what we thought was some sort of fossil tooth–we sent it to at least three or four different people, until finally it was discovered that it was actually just a really, really rock-looking bullet.