I have owned Canon camera systems for the past 27 years. For the first 21 years I did not have to send a single camera in for warranty repair of a manufacturing defect, and I did not hear about others experiencing problems. The initial quality of these cameras was so good that you could simply buy them and use them without extensive testing. Then came the EOS 3 with its built-in 2/3 stop overexposure bias. Canon built many cameras with this defect and did not recall cameras or publicize the problem when they realized what happened. They simply let owners find out on their own by ruining images. Since then it appears that receiving a new camera with a manufacturing defect became far more likely. Quality control may actually be more strict than it used to be, but cameras have become much more complex. More complexity means more defects. I have no statistics to fall back on here. What follows is based on defects found by a group of three photographers in their own new Canon cameras. The number of defects experienced by such a small group is a real eye opener. All of the defects that follow were addressed by Canon Professional Service (CPS). Here they are:
.
An EOS 1V randomly damaged film when a new roll was loaded. Damage occurred on perhaps one out of ten rolls. Factory service found no problem but replaced the camera back. The problem never occurred again.
.
An EOS 10D showed stuck (white) pixels in images taken by the camera. Factory service corrected this problem by a process that is essentially in-camera cloning.
.
An EOS 10D focused inaccurately. The camera always focuses at a point that is slightly closer to the camera than the actual object selected. This is corrected by returning the camera to factory service for adjustment.
.
An EOS 1D Mark II exhibited fine horizontal lines in its images. Image sensor replacement is the fix for this problem, but the defect was found soon enough to exchange the camera.
.
An EOS 1D Mark II exhibited a slightly dark crescent moon shape that covers about half of the frame. The effect can be seen only on solid colors, like a clear blue sky or when photographing a solid colored wall. I do not know whether the owner has fixed this yet. My guess is that the image sensor will need replacement. This is the only defect in my list that was not present when the camera came out of the box. Instead, it developed over time.
.
An EOS 1Ds Mark II shows several “dust spots” in its images, but no amount of sensor cleaning will remove or change the spots. The spots are randomly shaped and roughly nine pixels across. Like dust, they are invisible at wide apertures and sharper at small apertures. Factory service indicated these were within Canon’s specification, and they could not guarantee a replacement sensor would be any better. In fact, they said it could be even worse. This camera had a "thorough sensor cleaning" by Canon factory service, but it came back with more spots than I have ever left on an image sensor after a cleaning. 

.
An EOS 1Ds Mark II showed a long translucent mark when looking through the viewfinder. The mark did not appear in images. Cleaning the mirror and focusing screen did not change the problem. This problem was fixed by Canon factory service, by replacing a defective focusing screen.
.
What does this mean for you? It should be obvious that defects are not as uncommon as they used to be. Try not to buy a new camera immediately before an important shoot. Allow enough time to find problems and get them corrected before using the camera for important work. I would encourage meticulous testing of any new camera as soon as possible. If defects are found within the first week or so of ownership, many shops will exchange the camera. This lets you avoid  the hassle and shipping costs of returning the camera for service. Still, some subtle problems are not easily noticed, and some occur only after the camera is in service for a while. If you’ve had the camera for more than a couple weeks before you discover a problem, factory service is the only option. Canon factory service is generally good, and defects are usually corrected in a single visit.
.
Happy shooting!