There is a famous saying from the military world stating that: “No plan survives contact with the enemy.” At least one blogger has used the paraphrasing saying: “No plan survives contact with the data.” A good read by the way.
Like most famous sayings also this phrase is simplified from the original version. The military observation made by Helmuth von Moltke the Elder is in full length: “No plan of operations extends with any certainty beyond the first contact with the main hostile force.”
Translating the extended military learning into data management makes a lot of sense too. You may plan data management activities using selected examples and you may test those using nice little samples. Like skirmishes before the real battle in warfare. But if your data management solution goes live on the full load of data for the first time, there most often is news for you.
From my data matching days I remember this clearly as explained in the post Seeing is Believing.
The mitigation is to test with a full load of data before going live. In data management we actually have a realistic way of overcoming the observation made by Field Marshall Helmuth Carl Bernard Graf von Moltke and revisit our plan of operations before the second and serious contact with the full load of data.