I was experimenting with tick data in a table over 1GB in size.
There were frequent occurrences of series of ticks several points off the correct price. Additionally I remember seeing apparent bifurcations of the price (wild oscillations between two prices on alternating ticks), which I presume to be due to the prices for two successive contracts appearing together in the data.
The volume of data was far too great to cleanse manually and the nature of the aberrations was difficult to code around. It became rather apparent that there was layer-upon-layer of faults in the data - trying to fix it was like battling the hydra...
Trading signals are highly sensitive to such faults in the test data and the erroneous trades they generate are sufficient to have a very major impact on your results.