It appears that there is some bug in contourplot (Maple 7 linux). The combination of variable range for y, ﬁlled=true and explicit contours produces a large red triangle (side 10^14) for (x^2+y^2)^(1/4). Changing the exponent from 1/4 to 1/2 or 1/3 yields the expected plot. 1/5 and 1/6 also produce triangles.
This plots nonsense:
These plot something reasonable:
The bug seems to be ﬁxed in Maple 8.
I get the same bug in Maple 8 (U. Klein)