Rik Mayall does not receive a credit for his sole appearanced in the first series; his character, Mad Gerald, is credited as having played himself.
The 1998 special sketch contains a peculiar anomaly: whilst it is set in 1680 and features King Charles II, Lord Blackadder refers to his soon-to-be status "if Cromwell has his way". However, Oliver Cromwell executed King Charles I in 1649 and himself died in 1658, a whole 22 years before the scene is set.
The rotten borough of Dunny-on-the-Wold that appears in Black Adder The Third's [i]Dish and Dishonesty[/] was based on Dunwich, a real-life rotten borough that was underwater.
Source: No Such Thing as a Fish: Episode 168 - No Such Thing As Lord Cauldronhead