poisson: Some convergence issues

In this note we identify and illustrate some shortcomings of the poisson com- mand in STATA. Specifically, we point out that the command fails to check for the existence of the estimates and show that it is very sensitive to numerical problems. While these are serious problems that may prevent users from obtaining estimates, or even produce spurious and misleading results, we show that the informed user often has available simple work-arounds for these problems.