It seems that maple6 (trial version) cannot calculate contours correctly.
Try the phase space plot for the simple pendulum (m=l=1, g=10):
In version 5.01 we observe correctly that beyond the 5th contour we have rotations (rather than oscillations) as solutions. In Maple6 all 8 contours are (wrongly) shown as closed.
It is corrected with Maple 6.01. (U. Klein)
For a more dramatic example of this bug, try
This bug aﬀects the contourplot of any expression f where denom(f) is not 1. What is returned is a contourplot of numer(f), not f.
The culprit is in line 133 of `plot/iplot2d`:
It can be ﬁxed by replacing numer(F) by F in that line.