It looks like my site hit an imaginary limit of 400 daily impressions.
What is this limit based on? How can I "break through" that limit?
Google's webmaster tools shows this data with only one or two significant figures. Because of this, graphs often exhibit this type of appearance. You also commonly get graphs that look like stair steps.
Here is a list of values you are likely to see on your chart (but never numbers in between):
When it says that you got 400 impressions, it means that you got around 400 impressions. It could actually be 360 to 450 impressions or 400 to 500 impressions, depending on how they do their rounding. Four days in a row that got impressions in that range wouldn't be uncommon.
Here is a screenshot from my graph that shows something similar
I believe that Google does this so that they can store this data very efficiently. Because they don't have much accuracy, they may only use 8 bits to represent all positive integers.
EDIT: Google is announcing that this graph will no longer be rounded and bucketed like this. In the future it will be more accurate.