Moving on to the frame, be sure and check the pins. I've seen a number of broken pins, but the pistol was still functioning. The pins should be more or less flush on both sides. If not, a piece may have broken off and fallen out of the hole. Also, make sure the spring is okay under the slide lock. You can feel the tension when you pull down on the ends of the stop. When that spring breaks, the stop falls out to the side and the slide itself starts to head downrange. Also check the spring tension on the slide stop lever by gently lifting up on the lever (just enough to feel the tension). It can easily be damaged by improper assembly. If it is, the slide will usually keep locking back as the lever flips up during recoil. If it happens in the field, at least you can push it down and keep shooting. Visually check everything for cracks and/or wear. I've seen frame rails crack, but usually the gun keeps working. Take a close look during cleaning.
The most common problem I've seen (and it's not all that common), is a broken factory coil style trigger spring. I have heard of NY trigger springs breaking, but I've never actually seen one. The coil springs seem to be less durable. If the spring (it is actually the "trigger reset" spring) breaks, the trigger will remain in the rearward position when the slide cycles. If you're in the middle of a gunfight, that's a huge problem. So here is a gunfight survival tip for you: To keep the pistol running, hold the trigger to the rear and hand cycle the slide. Then release the pressure on the trigger and it will reset. You can keep firing that way, as long as you maintain rearward trigger pressure until after the slide has cycled. Actually, you might not even notice it in a gunfight, unless you let off on the trigger pretty quickly. But it may happen, and now you know how to keep it running.
Now, a few words about magazines. The most important words are: Don't lubricate the magazines. Like the firing pin channel, solvents or oil will only attract and hold dirt, causing feeding problems. You should disassemble the mags and clean them at least after each shooting session. Periodic cleaning, even if you have only carried them around for a while, is a good idea. The easiest way to remove the floor plate from the magazine is to use a 3/32" punch (the same size as the Glock armorers tool). Use the tool to push in the button on the floor plate and the insert will move off to the side (inside the magazine). Then use the tool, still in the hole, to carefully start levering the floor plate off. Be sure you "capture" the spring as the plate slides off. Then you can remove the spring and follower and clean the inside of the magazine tube with a clean, dry cloth or magazine swab. Wipe off the coils of the spring, too. If you have a new spring handy, check the used one against it. If it is more than two coils shorter than the new one, change the spring. Once you have it all clean and dry, it can be reassembled. A lot of malfunctions are magazine related. For example, if a cartridge feeds high and hangs up on the top of the chamber, the magazine lips may be too far apart. If the cartridge nose dives into the feed ramp, the lips are probably too close together. Change magazines and try again. Well maintained magazines can prevent all sorts of trouble.
Finally, a little about lubrication. Some folks have noted that people seem to over-lubricate their handguns and under-lubricate their long guns The only points on a Glock that need to be lubricated are the slide rails/frame rails, the outside of the barrel, the barrel hood or the corresponding wear point on the inside top of the slide, the barrel lug and, most importantly, the connector (that hook-like thing that sticks up next to the right rear frame rail). Just one drop only at each spot, please. Anything else is wasted.