While searching on Google, I just found somebody who has the same issue : http://www.mail-archive.com/lmms-devel@lists.sourceforge.net/msg00469.html

I'm running on Ubuntu 10.04 64 bits with LMMS 0.4.8 and still have these notes which don't stop. Is this specific to the 64 bits version ? Is there a solution to prevent from this bug, could it be corrected on the 0.4.9 version ?

Thank you !