Ya know, with Monty Python, I dont think you can really have a slipup. NOTHING in there movies are serious enough that a slipup would do something. A slipup in there movies would only make it better.
Not only that, Mr. T, but when Monty Python made this film, it was their first one ever. None of the productions companies would endorse them because they thought the movie would be a flop (HA! Now there's one for the record books!). All in all, the 6 Pythons only had a budget of about 100,000 dollars to make this film, which, when you count hotels, car rentals, food, salary, etc. is not a lot of money. Therefore, they probably didn't have the chance to hire a proper crew or anything. I mean, come on, in terms of seeing the microphone, there have been a lot bigger screw ups than this one, especially in the most recent movies when this happens, and you have the option of computer graphics.
You're right about the small budget there. They were initially going to use real horses, but they turned out to be too expensive, so they resorted to coconuts instead. It's fantastic how they managed to use them as an important part of the plot, though. "What... is the average airspeed velocity of an unladen swallow?" "African or European?"
They weren't arrested while filming the movie. They were charging down a slope at Castle Arrgh and the police came up and arrested the lot. The officer jams his hand into a news camera (I assume).