I ran across this in a service call the other day. The pipe coming from the lower left of the picture is a main line, which picks up a basement inlet then makes a right turn and continues on to the unit. Not only does the tee drop out of the main line at a 45 degree angle (filling the basement inlet with dirt), the pipe from the basement inlet makes a U-turn, forcing the air (and dirt) to do the same thing.
Here's the after shot. Not only is it better for the airflow, it avoids dirt from the main line ending up sitting in the basement inlet (and it took one less fitting than the prior arrangement to install).