TYPO3 Core

If you have tt_products installed, then the install tool also suggest this.

ALTER TABLE sys_products_orders DROP KEY uid;

This is a bug.

A very similar bug in the Extension Manager:
I have imported the whole database from another site. The tables are all there containing all contents. But the EM says:

ALTER TABLE static_countries ADD UNIQUE uid (uid);
    ALTER TABLE static_country_zones ADD UNIQUE uid (uid);
    ALTER TABLE static_currencies ADD KEY parent (pid);
    ALTER TABLE static_languages ADD KEY parent (pid);
    ALTER TABLE static_taxes ADD KEY parent (pid);
    ALTER TABLE tx_staticinfotables_hotlist ADD KEY uid_local (uid_local);

Resolved, no change required:

- All proposed changes are key indexes only, no dropped fields.
- In case of fe_users there's a standard key in 4.2 on pid/username named parent.