Yosemite won't empty trash
I infrequently have an issue where something is locking whatever item is in the Trash.  I reboot, and 10 times out of 10, I'm able to empty it.

For example, I save a screenshot to my desktop, then send it as a Mail.app attachment.  Once the mail is sent, I try to Trash and empty the screenshot.  I am unabvle to do so until I quit Mail and empty Trash.