A long story, but in brief:
For one his of columns, Jacoby rewrote an anomymous email circulating about the heroic fate of the people who signed the Declaration of Independence (killed, tortured, property destroyed). The email was full of errors, some of which Jacoby corrected but some he missed. His article had the same form as the email, if slightly different content. For this transgression, the Globe suspended him four months.
For a full account see Chatterbox at Slate.
Jacob