This gallery shows some pictures of bad solderings - they should prevent you from making the same errors (feel free to add even more pictures here!)

Don't be shy if your soldering isn't perfect. We gotta learn sometime, and trying to hide it if you get embarrassed doesn't help anyone. :)

1: The core module has some badly soldered connections(loose), which could lead to failures. There are also missing solderings (e.g. 33pF cap below crystal). It makes sense to solder each pad again, the Soldering guide in the Wiki gives some useful tips
2: The legs of the caps should be cut, otherwise they can cause a short circuit which can damage the PIC!

3: This seems to be a bad ground connection directly at the 78xx - it can lead to the wrong voltage. Here the same: just solder the pads again, it will help
And same as above: the long cap legs need to be cut, the danger of shorting out is too high!

4: This board has too much solder. One experienced midiboxer cleverly said when he saw this pic, “Solder is not glue - more is not better”. This high-res image is zoomed out to 200px wide; zoom in for excellent detail.