The first computer I tried to build was a complete disaster. This was several years ago. I was reusing the case, hard drive, and mobo from my previous computer, a store bought one. So I guess it was more of an upgrade than a new build. Anyway I ordered all my stuff off newegg, even got one of those static wristbands. I thought I was ready, and had gotten all the right components that were compatible with each other. So I put everything together and thought it went well, but the computer wouldn't work. I opened it back up and discovered that a corner of the CPU wasn't in all the way.
Next I did what comes naturally when something needs to go into something else and forced it

. As it still wouldn't go in I took it out and looked at the bottom to see what the fuck was up, and lo and behold the shape on the bottom of the CPU and the shape on the socket didn't exactly match up. At the time I thought "hey my old CPU is called intel and so is this one, so of course it will work!". Nevermind that my old CPU was a socket 478 Celeron D and my new one was an LGA 775 Pentium. Eventually I read up some on the other computer and figured it out. FFFUUUUUCCCCKKK.
So I then ordered a new mobo! And the sockets matched and I was happy. So I got the new mobo and put it in and god dammit the cpu still wouldn't go in the fucking thing. Then I got to looking closer at the CPU and realized some of them damn little pins were bent from my previous forcing session. I returned it to Newegg for a replacement.
I got it and put everything together and this time it fucking worked! Well, the computer worked anyway, but the Windows installation I had on my old hard drive didn't. Thanks to the new mobo Windows was freaking out thinking it was pirated or some shit. Since the version I had was a factory OEM with no disk, I couldn't do a repair install or anything. At this point I'm just pissed and wishing I had never spent my money on this shit.
Anyway long story shortened some I ended up downloading and installing OpenSuse on it (Also my first time using linux, which really got me into it, so maybe in a weird way this was all meant to be), but a couple days later the entire computer crapped out and wouldn't start. To this day I'm still not sure exactly what it was, but I suspect the PSU since I kinda went cheap on it. But at that point I was just fucking tired of it so I returned what parts I could and got a laptop.