If you have experienced problems with your Mac and have had to take it in for servicing, be sure to check your system's serial number when you get it back, especially if the service required a logic board replacement.
While most of the time applications and services you use in your Mac will be relatively independent from the hardware setup, there are some instances where specific software may matter. Recently a hint was posted on MacWorld that discusses the reliance of Apple's FaceTime technology on the hardware of the system, where people who have had logic boards replaced may not be able to connect because of an incorrect serial number following the repairs.
Other programs that use encryption and similar technologies may also show incorrect behavior if these also rely on the system's serial number.
When logic boards are replaced in a system, the technician is supposed to update its serial number to match that of the Mac's chassis. If that is overlooked, some services, such as FaceTime, may not work properly. Therefore, be sure to check your system's serial number if it has been repaired to ensure that it matches the one printed on the chassis of your system.
There are a couple of ways to check your serial number in OS X. The first is to choose "About this Mac" from the Apple menu and then click the gray text of the operating system version, which should toggle this text between displaying this version string, the build number, and the serial number.
The second approach is to click the More Info button in this window, which will open the System Information utility, where you can see the serial number listed in the hardware section.
If the technician forgot to update the serial number, it may appear blank, or with zeros, or something else besides the one that is printed on the bottom of your system's chassis. If this is the case, take your system back in to have it properly updated so your programs and system services will work.