When many components request repainting at about the same time, some do not get repainted properly. See the attached test program. It displays some number of components (I have tried buttons and combo boxes, both show the problem) and allows the user to enable or disable all of them in one operation.
The behavior is timing dependent. On my system, with 10 components, repainting is correct, but with 15 occasionally one component will not be updated, and with 20 or more several will not be updated. It seems random which components do not get updated. Also, I have observed that running under a debugger with print-only breakpoints can make the problem go away. I have observed the problem with Aqua LAF and Nimbus LAF.