For all the packages I mail (and receive), I can't complain. However, here are a couple of interesting ones:

(1) At my local P.O., one of the clerks continually tells me that a bubble mailer is not thick enough to get Delivery Confirmation. Every other clerk in the same P.O. takes them with confirmation without a problem.

(2) A while ago, when I was sending a package to the UK. I listed the item being sent as "poker chips". I was told that this was illegal (as that was considered the same as currency) and the clerk refused the package. I left the line, grabbed a new customs label, listed the same items as "casino memorabilia" and the same clerk accepted it. I now use "casino memorabilia" in my description.