There is a theory that the photograph given to Hurley, that is supposed to serve as proof that Dave is a figment of his imagination, may actually be a fake. Gasp! Shock! Horror! Etc.
Before we get into this idea, it should be pointed out that this entire issue is explicably due to continuity errors. For the record, that’s what I believe. And there's proof enough for me presented further on. The point of the photograph is that it proves Dave was not there when Hurley thought he was. In short: Dave is a figment of Hurley's mind.
That being said, let’s look at the discrepancies anyway.
So here’s the moment when the photograph was being taken (which I screen-grabbed from my own DVD so I know it's as close to the 'flash' of the camera as the picture gets taken as you can get). . .
Allowing for the slight change in perspective from the TV camera shot to the actual camera angle, there are numerous errors between what was in the live-action frame and what was captured in the photograph. To wit:
1. The arrangement of the celery on the plate.
2. The placement of the Connect 4 pieces and small cup is not correctly aligned. When the photo was being taken at least one of the Connect 4 pieces was underneath the edge of the plate, whereas in the photograph they are quite some distance away.
3. The person(s) sat behind Dave at the table. Potentially, from the angle the photograph was taken compared with the angle we saw the photograph get taken, the man sitting on our right at the table could have been missed off the picture, but the guy sitting to our left of the table should definitely be visible behind Dave's shoulder. He's not. (Is this guy also a figment of Hurley's imagination!?)
4. For the really pedantic, Hurley's grin isn't as big in the photo and he's not leaning in as much.
Of course, as stated, these are all surely just technical issues from the real world making of the show we call Lost - but if you want to get all conspiratorial then you would have to theorise that the doctor was involved in producing a fake photograph to dupe Hurley into believing he had an imaginary friend, that Dave was indeed a real person and, therefore, his appearance on the Island is laden with even more mystery.
Ah, but wait. We’ve got objective proof of the truth.
We do get another perspective on the whole scene. Libby. Here we see her point of view of the moment when the picture was being taken and here we can clearly see that Dave is not present.
Mind you, from this angle, it doesn't look like there are any Connect 4 pieces on the table. . . And where's the cup!? CONSPIRACY! It's a COVER-UP! Dave was really there all along! NURSE! MORE DRUGS!
Ahem. Well. Anyway. I guess that's the end of that. You can get on with your life now.