That's a really broad question! A huge range of problems can occur during synthesis, and the problems are all different depending on which synthesis tools you are using, and your personal experience level.
When you say "error" are you talking about error messages from the synthesis software? Or do you mean that the synthesis process runs smoothly, but the hardware doesn't behave as you expected?
The best resource for understanding synthesis problems is usually the manufacturer's web site. For example, Xilinx has an extensive Answer Database that has helped me solve many of the puzzling problems that I've encountered using Xilinx FPGAs.