Not all contacts are made the same even in the same pack. I have no idea what the acceptable level of failure is for contact manufacturers though.
Let's assume that you abide by all the usage instructions; that is, if they are monthly, weekly, or daily, you change them at that correct interval. Also, note that some contact lenses will have a longer life span with certain contact lens solutions. Off the top of my head, Air Optix is one of these brands. With this brand, you should be using Pure Moist I believe. Just following the boxes cleaning regiment will not suffice if you are using the wrong solution with the lenses.
Now, let's suppose you are doing everything correctly. Unfortunately, some lenses will not be up to par (such is engineering and in life not everything is perfect). Therefore, this set of lenses will become less breathable in a shorter amount of time. If you are the unlucky soul who perpetually gets the dud box, there can be negative side effects. One side effect is from ischemia which was mentioned in the comments. When the eye is starved of oxygen, blood vessels will begin to grow on the eye in order to get oxygen rich blood to the tissue and over time will cause Corneal Neovascularization.
The scenario I described is a long shot but it cold theoretically happen although it is very unlikely.
Other side effects could be that you get some kind of dirt or grim in your eye that gets under the contact lenses. This foreign body could end up scratching the cornea.
However, if you are diligent in following your eye care instructions, use the correct contact lenses solutions, and use common sense, more than likely, you will not have any problems from wearing contact lenses.
- Contact Lenses cause Corneal Neovascularization
- Corneal Abrasion