From a reader:

Here’s an assertion which I don’t have the time to check today (I’ll see if I can find some stats to either prove or disprove this assertion tonight):
“For an accelerating economy, revisions are generally upwards. For a decelerating economy, revisions are generally downwards.”
This makes some abstract sense if you assume a) that GDP estimations are measuring “things” that are coming into and out of existence and b) your best first estimate of the rate of change in the current number of those “things” is the measure from the previous quarter. For instance, consider the creation of companies during an economic cycle.
Now estimating the number of companies in a large economy can be a pretty difficult task. Assuming the change in number of companies is the same as the previous quarter is not a bad first estimate. However, for an accelerating economy, that leads to an undercount in the first estimate as the acceleration has led to a faster growth rate. Similarly during decelerations, that first estimate leads to an overcount.
Now, I’m an engineer, not an economimst, (Dammit Jim!) so the assertion might well be wrong, but it does gives the GDP economists an “out” for being off by 10% for some scenarios.