One example of such an idea is that the 20th century was especially violent. This is based on recollections of two world wars, enormous progress in arms technology, and comparison with the two previous centuries. However an analysis made by modern historians suggests that the loss of life due to violence (as a percentage of population) steadily declines if we look at long periods of history.
References: Lawrence Keeley, War Before Civilization The Myth of the Peaceful Savage. Oxford UP, 1996. Steven Pinker, The better angels of our nature,
Viking, 2011. (There is no consensus on this, see
The Blank Slate, Modern Denial of Human Nature for exposition of various points of view).
Here is a little specific example from Pinker (he is talking here about criminal violence):
When I surveyed perceptions of violence in an Internet questionnaire,
people guessed that 20th-century England was about 14 percent more violent
than 14th-century England. In fact it was 95 percent less violent.
Another common misconception is that people started to influence
significantly their natural environment only with the beginning of the industrial era.
This is disproved by many examples both from history and pre-history. Examples: extinction of large mammals in Australia and Americas in pre-historical period.