1. Blob around Extruder
What is it? Item comes off the print bed midway, sticks to the nozzle, and envelopes the entire extruder as a blob of goo which once hardens may make it impossible to remove from your printer.
How to fix? Watch your prints from start to finish (often this is impossible and impractical to do) or try to set up an IP webcam to catch instances where this is happening.
2. Warping
What is it? Warping is when you have a large part to print and the first layer which originally adhered so well begins to warp at one corner because of uneven cooling.
How to fix? The obvious fixes for this is to use a heated bed (check), use a heated enclosure (check) and print with a good brim or raft (check). Unfortunately even my rafts still continued to warp, so the main solution I used to fix this is to avoid the edges and to avoid overcrowding the print bed. I say solution but really it was just a temporary coping mechanism to try to get on with printing despite occasional warping.
3. Gap between parts
What is it? This is where someone has designed a part with an error and failed to check if the slicing is alright, resulting in this mess.
How to fix? Check and double check the design of all parts before printing! Look through every single layer if you have to to ensure that the thread continues and doesn't just stop for a few layers.
4. Insufficient supports
What is it? Many a time I would see a file that did not seem to have sufficient support to keep the object from falling over. But since I wanted to make a teaching point of this, I let the item go to print so we could all experience the failure together. Now that I have observed several hundreds hours of prints, I find that I am pretty accurate at eyeballing a print and determining where a print lacks the supports for it to complete without falling over.
How to fix? ADD MAXIMUM SUPPORT. Don't just do what you think is the minimum, because that is almost certain to end in tears. Add the maximum amount of support needed if possible and accept the fact that the only way to get some objects printed is to allow it to take the longest possible time to print (with all the additional support)
5. Ugly Overhang
What is it? This is where you knew the printer would make a squiggly mess of the overhang yet you let it go on printing.
How to fix? Like the previous support issue, you just have to add more supports than you think you need. Or slice the item into half and lay flat the two halves to print.
6. Flipped Normals
What is it? If your 3D model has errors or flipped normals in any portion of itself, it may be translated into a weird shell of itself if you forcibly slice it into gcode.
How to fix? FIX YOUR MODELS COMPLETELY BEFORE SLICING THEM.
