All things can break. For some reason, then tend to fail at the most inappropriate times.
Crosshair reticles generally work best when still in the scope. The story behind this picture is that the owner of the cheap scope found it broke and the reticle turned in the scope. He attempted to disassemble and fix it, but only managed to accomplish the disassembly part.
AR15 bolts tend to give the best reliability and function when installed so the extractor and ejector can kick the round out of the ejection port. This BCM bolt was milled wrong allowing the owner to install it backwards causing a malfunction after the first shot.
A great deal of shooters are have been purchasing old surplus ammo. Often this is the way to get good cheap ammo. However some of this old ammo has been stored poorly or has various issues like severe corrosion, or cases cracking when the firearm is fired. If your cases are cracking like the one above those hot escaping gases can erode your chamber and potentially causing future issues.
As always, when things work correct we rarely give them a second thought but when things break they sure get our attention.