It happens very often when there is no text-ui opened, and new message coming, but kinds of random. Here is an screenshot demonstrate this. http://wstaw.org/m/2012/08/12/plasma-desktopqn1574.png The expected result is tray icon have correct size and shown as envelop. Reproducible: Sometimes
Confirming as it happens for me too. I remember showing that to Plasma people in Pineda and they were rather clueless.
Has anyone seen this since running KDE 4.10? the sys tray has been rewritten.
I have not. No update for 3 months, so closing. If you will spot this again, please reopen stating your KDE version as well as KDE Telepathy version.