It can be tempting to remove all records from a table or just set one column to NULL in a whole table to get a test working quickly.

But we should better avoid that, or replace it by something less drastic (if reasonably possible).

An example from Members.t:

$dbh->do(q|UPDATE borrowers SET lastseen=NULL|);

Surely you should not run such tests on a production db. But when someone forgets a rollback here or there, you might have some troubled data on your test system, and often needless.
Note that I still often see test branches or item types appear in my test data after spending some time on a test db with master (running some tests).