To upgrade your MacBook's RAM or hard drive, or otherwise work on its interior, you'll need to remove the bottom of the case. This is fairly simple to do with the appropriate screwdriver, but when you reassemble the system, even if you do so with expert precision, you may find that a screw is loose or even missing after a while.
The screws holding the bottom case together are fairly small, and while you can tighten them significantly, slight flexing of the bottom cover and chassis (unavoidable on portable systems) may work them loose over time. Additionally, since the screws are so small, many people may not want to risk stripping them so they leave them relatively loose, which can also contribute to them coming undone.
If you're lucky, you'll detect the loose screw; maybe it'll catch on something as its head protrudes beyond the plane of the case or you may detect it by feel. If not, then you may at some point find an empty hole where the screw used to be.
To avoid this from happening again the next time you reassemble your MacBook, use a small amount of thread cement on the screws. If you look closely when disassembling the system, you may notice a small amount of a colored film or pastelike substance on the threads of the screws. This is a special cement that ensures small screws such as these stay put when tightened.
Be certain that you use the right kind of cement when you replace it. While any glue will hold the screw in place, you're probably going to want at least the option of taking it out at some point, so avoid using Krazy Glue, epoxy, or other household adhesives. Get a cement made specifically for this purpose; it should be labeled "Thread Locking Cement" or something similar and is usually available at hardware stores.
To apply the cement, you usually just have to lightly coat the threads of the screw before tightening it when reassembling the system. Apple's case screws are relatively small, so this may be a bit of a challenge, but you can also apply cement to a toothpick or paperclip, and then dab a little in the hole, instead of trying to get it on the threads beforehand.
In most cases, using too much cement will not hurt at all and only results in an unsightly glop (but it can be cleaned up with a quick wipe or two); however, you do want to avoid dripping the cement into the system.
If you do find a screw missing from your system, it is not the end of the world. The bottom case will still hold together nicely with the remaining screws and built-in snaps, but you might want to consider replacing the screw. Unfortunately the screws Apple uses are not readily available at the local hardware store, but you can purchase used ones from places like iFixIt or eBay. Alternatively you should be able to bring the system to an Apple Store and ask technicians to pop in a replacement.