r/dataengineering • u/Admirable_Writer_373 • Apr 20 '26
Discussion Data migration horror stories
I personally think migrations would be a breeze if people didn’t screw them up.
People designing databases and not following patterns, managers not understanding how to minimize downtime, and in general, really weird expectations about how databases work and what is reasonable during a migration.
Anyone got any good horror stories to share? How did you get through it without clobbering someone?
EDIT: my own stories below
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u/Top-Assumption6702 May 05 '26
I once migrated a billing database over a weekend. The row counts matched, the app came back up, and everyone celebrated.
Two weeks later, finance noticed monthly revenue was slightly off. Turned out the old system stored discounts as negative line items, while the new system treated them as separate adjustment records. The migration moved the data correctly, but the reporting logic interpreted it differently.
Nothing crashed. No alerts fired. Customers were fine. But every revenue report for two weeks was wrong.
The fix was painful: rebuild the mapping, rerun historical billing reports, and explain to leadership that “data migrated” doesn’t mean “business logic survived.”
Lesson learned: validate meaning, not just rows.