As already mentioned, the causes for some problems are hard to track down. In general, these problems only occur occasionally and often are difficult to reproduce and analyse without adequate tools like I2C monitors.
Tracii XL 2.0 is an I2C monitor which can detect I2C error conditions that are very hard or even nearly impossible to find with usual oscilloscopes, it helps to analyse them by triggering oscilloscopes or recording digital and even analog traces of SDA and SCL.