Below is a list of possible problems a keyboard may have.

Keyboard key problems:

Broken keytop
Keytop is the surface or upper part of keycap. It may be broken by extensive and violent pressing. A broken keytop may wound your fingers. It is not safe to use and have to be replaced.
For some keyboard, especially game keyboard, there are replacements for those frequently used keycaps like ADSW and arrow keys. Well, broken keytop is rarely happens, since the rubber spring under the keycap will buffer pressure.

Scratched keytop
This means the keytop has been scratched by mistakes. It can still be normally used, but it will not be good for keyboard appearance. Scratched keycap can be avoided by carefully handling. Moreover, properly packing including inner packing and outer packing will reduce the chance of being scratched to minimum.

Obscure letter printing
This mainly happens in keycap with pad printing or silk screening. It will have bad effect on keyboard appearance, though it may not bring too much trouble for those used to blind typing.

Keycap misplacing
Keycap may be misplaced during assembling process. For instance, in US layout, “O” is placed on left side of “I”. That is misplacement. The two keycaps should exchange position.

Hard key
Hard key means a key hard to press down. It differs from struck key. It is mainly caused by too much and unnecessary fraction between keycap stand and the slot holding the stand. Hard key can be relieved by adding lubricating oil between the stand and slot.

Manufacturer will do the lubrication while producing the keyboard. If this not done well, the key may get hard. Hard key can also be caused by keyboard molding problem. Some keyboards, though with enough oil on keycap, may still have hard keys.

Struck key
A struck key means a key hard to press down, or will not bounce back after that. To be exact, struck key happens mainly because of followings:

1) keycap edge untrimmed.
Most keycaps are made by plastic injection (for those ABS keycap). There may be blurs on edges, which need to be trimmed or cuts before placed on keyboard. If not, the two neighboring keycap make got struck together. There may cause two results. First, keycap may be hard to press; second, after pressed down, it can not bounce back. The latter may be called as sticky key too. Keycap is sticky too membrane beneath and result in consistent signal inputting. For example, “aaaaaaaa…..” 

2) broken rubber spring
As mention above, if the rubber spring can not bounce back, keyboard input will go wrong. So, broken rubber spring can also cause keyboard malfunction. Even if it is not broken, a rubber spring may not be working because of aging.

Other keyboard problems:

Damage membrane
The membrane is a layout of rubber sheet which transmits signal while in the same time buffers pressure from keycaps. Too extensive and violent pressing on a same spot may result in its damaging. That is why tender press on keycaps is suggested while typing.

Broken contact or PCB
There are contacts printed on PCB for all keycaps. They could be damaged when too much pressure is applied to. To determine if a contact is broken, use a millimeter to trace the circuit. There should be resistance if it works well.

Besides those problems, even a normal keyboard may sometimes not recognized by computer. Plug the keyboard in and out for several times to see if it can be solved. Also, check if there is loose connection with your pc.