Even though aptitude and synaptic are both front-end to the APT (and I myself stick only with apt-get), peculiarities in each of the available dpkg/APT frontends seem to cause problems and confusions when they are used interchangeably. Beginners especially -- who tend to do copy & paste more often, thereby executing apt-get and aptitude commands more or less identically -- may find some situation puzzling.
Please list the possible problems that might arise when they are mixed up and tips to avoid when one has to rely on multiple packaging managers.