One of the most fun areas to collect has been the stamp bulletins. These came out in several different formats over the years, and they are numbered, so it is relatively easy to tell what is missing.
The first format I started collecting was like this one to the left. It is a color folder that has pictures of the stamps on the front, and some description and issue details inside and on the back. These were issued starting in 1959, and appear to start only with #4. There are 3 issues with no number on the front, and I assume those were #1-3. I have not found any documentation to back this up, but no one I have talked to has ever seen #1-3. There is also a fourth issue with no number, that I believe is supposed to be #19.
There are also some varieties of these bulletins. Later issues came with both a blank front, and printed with the agency name, like this example here. I have several different agency names, not just the philatelic services one shown here.
These bulletins currently go through number 780, which was the August 2007 issues.
There are also a few errors that I have seen in these notices. Here is one such example:
Between the printing of the bulletin and the printing of the stamp, the value changed. Rather than issue a new bulletin with a new number, the same number was re-used. Thus, there are 2 bulletin #375's.