COPYING mention that the software is under GPL but the sources are under LGPL.
Fixed in revision 2892: * panel-plugin/*.{c,h}: Fix license headers. The source is under GPL, not LGPL. Fixes bug #3366.
panel-plugin/time-out-countdown.{c,h} are still under LGPL. Is it a mistake ?
Yes, that was a mistake. I just fixed it in SVN.