Using gold pressure plate as a counter. I dropped blocks on the pressure plate to see if counter functions correctly in accordance with table at http://www.minecraftwiki.net/wiki/Gold_pressure_plate. Here are the results:

Signal Blocks Confirmed in table
Strength Dropped
0 1 to 4 Yes
1 5 to 8 Yes
2 9 to 12 Yes
3 13 to 17 NO
4 18 to 21 NO
5 22 to 25 NO
6 etc

I was trying to set up a 20 minute daytimer using blocks that were dropped into a water sluice using sections of ice floors and signs so that the blocks travel time from start to end was 1 minute. At its arrival the block triggered a trip wire that caused a dropper to cause the next block to travel the sluice. The block triggering onto the trip wire then fell on to the gold pressure plate which was configured to avoid blocks escaping from the plate. All this worked except the count anomaly described above.

I then set up a simple experiment where I dropped blocks onto the pressure plate and watched how the signal changed incrementally with the number of blocks on the plate. This experiment confirmed the results shown in the table above.

As you can see, the critical signal increment from 20 to 21 blocks for the 20 minute day timer doesn't work. Twenty-one blocks does not cause the signal to increment. 22 blocks do.

I did not test the other increments to see if there was only this error.

(later)

I just tested for the 15 intensities off the gold pressure plate. Here are the results:
Signal Strength 1 is for 4 blocks (1 to 4)
Signal Strength 2 is for 4 blocks (5 to 8)
Signal Strength 3 is for 4 blocks (9 to 12)
Signal Strength 4 is for 5 blocks (13 to 17) !!!
Signal Strength 5 is for 4 blocks (18 to 21)
Signal Strength 6 is for 4 blocks (22 to 25)
Signal Strength 7 is for 4 blocks (26 to 29)
Signal Strength 8 is for 5 blocks (30 to 34) !!!
Signal Strength 9 is for 4 blocks (35 to 38)
Signal Strength 10 is for 4 blocks (39 to 42)
Signal Strength 11 is for 4 blocks (43 to 46)
Signal Strength 12 is for 5 blocks (47 to 51) !!!
Signal Strength 13 is for 4 blocks (52 to 55)
Signal Strength 14 is for 4 blocks (56 to 59)
Signal Strength 15 is for blocks 60 and up

Thus the error repeats itself at Strengths 4, 8, and 12. In these Strengths, it takes an additional block over the 4 shown in the table on the wiki referenced above to roll over into the next Strength.

Hope this helps...