I realized some lenses and cameras seem to have typical defects or problems

This is not a proof of wrong parts, could probably in some cases be special use. For example I have read about some big Canon EF super tele lenses with fungus (1, 2, 3 ) . This could probably be a result of a special coating, or more likely these lenses are more often used under humid enviroment than the cheaper lenses most amateurs have.

Here some first ideas about common problems, with some references. But one have to realize, this is not a scientific proof. Some people could write in different forums, or a defect lens could be sold to another one who writes about it, or one could write something even without owning the lens / camera...

 

Samyang 8mm f/3.5:
Focus inncorrect: 1, 2,

Samyang / Bell + Howell / Rokinon / Walimex / Falcon / Polar / Bower / Opteka / Vivitar 14mm f/2.8:
Distance scale not matching real distance: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13,
Bad build quality: 1, 2, 3, 4, 5,

Samyang 35mm f/1.4
Stuck iris: 1, 2, 3, 4, 5,
Wooly bokeh: 1, 2, 3,
Build quality: 1, 2, 3, 4, 5, 6, 7, 8, 9,

Samyang / Rokinon / Walimex / Falcon / Polar / Bower / Opteka / Vivitar 85mm f/1.4
Stuck iris: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
Wooly bokeh: 1, 2, 3, 4,
Build quality 1,

Tokina 12-24mm Pro DX
Broken ribbon cable - iris not working: 1, 2, 3, 4,

Canon EF-S 17-85mm IS USM
Broken ribbon cable - iris not working: 1, 2, 3,

Canon EF-S 17-55 f/2.8 IS USM
Much dust inside: 1, 2, 3, 4, 5, 6,
Mechanics: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,

Canon 35mm f/1.4 L USM
Fokus mechanics: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,

Canon EF 60mm f/2.8 Macro
f/2.8 Bokeh not circular: 1, 2,

Tamron 90mm f/2.5 Macro 52B & 52BB
Hot Spot an DSLR: 1, 2, 3, 4,

Nikon Nikkor AF 50mm f/1.4 D
Hot Spot an DSLR : 1, 2, 3, 4, 5,

Sigma 70-200 f/2.8 HSM
Points / bubbles in frontlens: 1, 2, 3, 4, 5,

Kiron / Vivitar 28mm f/2.0
Stuck iris: 1, 2, 3

Leica Elmarit 135mm f/2.8 R
Separation: 1, 2,

Older Schneider lenses
"Schneideritis" - bright dots on blackend lens surfaces: 1, 2, 3, 4,

Older Rodenstock Rodagon enlarger lenses
Separation / Delamination: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,

Canon EF 17-55mm f/2.8 IS USM
Dust: 1, 2, 3, 4, 5, 6,

Canon EOS 30D, 40D, 50D
Shutter button: 1, 2, 3, 4, 5, 6, 7, 8, 9,

Tamron 28-75mm f/2.8
Wooly / irregular onion ring bokeh: 1, 2,

Canon EF 50mm f/1.8
Front element pop out: 1, 2, 3, 4, 5, 6, 7, 8,

Canon EF 50mm f/1.4 USM
AF defect and sticky focus: 1, 2, 3, 4, 5, 6,

Canon EF 50mm f/1.2L
Focus: 1, 2,

Sony Zeiss ZA Sonnar 135mm f/1.8
Front element wobble: 1, 2, 3, 4,

Sigma lenses with ZEN rubberized finish
Zen finish gets sticky / peels off: 1, 2, 3, 4, 5,

Sigma 400mm f/5.6 APO 72mm filtersize
Bubbles / haze in lens: 1, 2, 3,

Sigma 600mm f/8 Mirror
Fungus (?): 1, 2, 3, 4, 5,

Canon AE-1, A1
Shutter squeak: 1, 2, 3, 4, 5,

Yashica FX-D, Contax 139,
Plastic leatherette defective: 1, 2, 3, 4, 5,

Minolta M-Rokkor 28mm f/2.8
White spots on / in front lens / coating: 1, 2,

Phottix / Pro Optic Extension tubes
Killing the fuse in a Canon EOS 5D camera: 1, 2,

At the moment I want not to list the very common defects like Canon EOS 5D mirror, Sigma iris problem with old lenses and new cameras and such.
Some of the references may not show the pictures to non-members, sorry for this.

There is a list off Lens Rentals with their 5600 lenses and the lens repair data 2010, 2011, 2012 there!

Please visit my other tinker work, or my DIY directory with ~1700 DIY links to other photo tinkering sites.