... you might be bitten by this Java bug rendering arcs as straight lines at large zoom levels.

It looks like a classic to me: in order to improve rendering performance, you approximate arcs with straight lines at small resolutions (if it's just 2 pixels big, nobody will be able to tell the difference). Except of course, when you end up doing the same approximation at a large zoom value - of course a 100-pixel circle looks different from a 100-pixel diamond.

Reported in 2005, still not fixed in current Java (we're in 2009 now).

Sun is really slow at fixing Java bugs.

See also a related Apache Batik bug report. Fortunately, this only applies to Java rendered graphics - SVG export, PDF, Postscript are all fine.