In addition to intentional judgment calls that lead to inconsistencies between products, there are myriad tiny design mistakes which are legion with 1394 products. Even among household names, dozens of common design mistakes are all too common.
For example, if your board layout includes differential traces which are different lengths, you can introduce skew. I.E. if the board in your product with the computer chips on it uses wires of different lengths, you can cause some of the audio, video, or data to arrive a little bit late.
Even more shocking (pun intended), many manufactures do not design their product to accommodate the right electrical current carried by the FireWire wire. Hook up a device providing the full FireWire electrical current and the poorly designed product will not be able to withstand the current coming over the FireWire cable.
The power problem would produce a more obvious failure than the board layout problem. Most of the common design mistakes are not so obvious. For example, manufactures sometime do not understand some of the information their product needs to provide when communicating with other products over 1394. They will leave information out or duplicate information in all their products causing significant confusion and poor performance.
These problems could have been significantly reduced if the 1394TA had a strong compliance program. That is great in theory, but difficult in reality. The first 1394 product emerged in 1995 but the first meeting to discuss a compliance program did not take place until 1998. Meaningful work did not start on a compliance standard until 2002. After 7 years of products shipping, the sense of urgency for testing products was very low and the TA was powerless to do anything about it.