The first one just needed a cleanup and some new thermal pastes. But afterwards, when I was test driving the laptop, I found the machine started to crash randomly - the OS was a fresh install. And later, I found whenever I put the laptop on the metal chassis on my PC (it was on the same table), it could trigger a kernel panic. Oh no, did I accidentally shorted something on the motherboard to the chassis? I reopened the machine and reseated the board but it didn't solve the problem. I also tried to reproduce the problem with a multimeter probe, but it didn't work. I got the final breakthrough while searching the web using the laptop while lying on a sofa - whenever my lap kicked the bottom cover, it would crash instantly!

The DIMMs came loose. Reseating it fixed the problem - the machine was previously dropped, which also explained the cracked plastic I saw on the battery.

Coincidentally, one day later, another family friend (with absolute no relation or knowledge about the former one) sent me a message, showing a MacBook with random screen corruptions. I said "you're out of luck, better to get a replacement from Apple." Later, I was also told that screen corruption was not the only problem - whenever a spot near the keyboard was punched, it would promptly crash! I couldn't stop laughing, "Okay, just send it to me, it's a 5 minute fix..." And yes, the machine was previously dropped by kids.

Nowadays, most MacBooks come with soldered RAMs. But if your machine has sockets, make sure to reseat the RAM and swap the slots first before declaring the death of your motherboard or your RAM.