Cigarette Burns
Here's a collection of screen-caps where the reel change markers are visible. The fact that these are evident on the DVDs I was viewing means, I guess, that these particular discs were sourced from less than ideal materials (theatrical prints, etc.) rather than whatever they might use in more ideal situations, but I don't really know enough to say for sure. I do notice that most newer DVDs, or newer films on DVD, lack these artifacts; perhaps they've been digitally removed, or, if the film is shot and projected digitally, they simply don't exist at all?
The example above is an actual cigarette burn through fabric - almost literally through the fabric of the screen, evoking Alice's looking glass - from David Lynch's Inland Empire.
The image to the left is from John Carpenter's Assault on Precinct 13, one of the leanest action films ever made, a fantastic, harsh film, riddled with startling violence -- even the marker here seems portentious.
This is Billy Wilder's The Apartment. I'm assuming that where the reel markers are an oval it means the film was shot anamorphically.
These two are from Jimmy T. Murakami's Battle Beyond the Stars.
Two pics from Vera Chytilova's psychedelic masterpiece Daisies. Interesting that here the markers are obviously hand drawn or scratched, perhaps a function of limited budget or technology? 
A couple from Toshio Matsumoto's amazing Funeral Parade of Roses. The image below is, again, like the Lynch image above, of a literal cigarette burn.
These two are from Lesley Selander's Flight to Mars, a real celluloid turd. One frame has the traditional "mechanical" mark while the other has what appears to be a smear of grease pencil.
Mike Nichols' The Graduate.
Radley Metzger's The Image.
Stanley Kubrick's Killer's Kiss.
Engel and Orkin's Lovers and Lollipops. These are my favorite -- apocalyptic, terrifying.
Monsieur Beaucaire, 1946.
Otto "Mr. Freeze" Preminger's The Man with the Golden Arm.
Michael Almereyda's Nadja.
Powell and Pressburger's The Red Shoes.
Arch Hall, Jr. is The Sadist!
Bob Hope's Son of Paleface.
Jack Hill's Switchblade Sisters.
Jaromil Jires' Valerie and Her Week of Wonders. These are odd: One frame square, the next circle.
Engel's Weddings and Babies.
Where There's Life, 1947.
Antonioni's Zabriskie Point. Fuck America!
The Marx Brothers in Go West.
