In the third paragraph of chapter 17, "Dr. Kemp's Visitor", is the phrase "The moon in its first quarter hung over the westward hill".

Yet in the first paragraph of chapter 18, "The Invisible Man Sleeps", is the sentence "Outside the night was very quiet and still, and the new moon was setting over the down."

There are two things wrong with this:

1) A new moon rises and sets with the sun; if a new moon was setting, it wouldn't yet be night.

2) The second scene is only about one to three hours after the first; the moon couldn't possibly have gone from first-quarter to new in that brief span of time, such a change takes about 21 to 23 days.