User sessions running an application in a seamless session are randomly being locked when they are pressing the “L” key.
Update to a version of the Online Plugin or Citrix Receiver that includes fix #259108.
This is a known issue with the older ICA clients.
“Pressing the letter "l" key in a seamless application can inadvertently lock the session.”
[#259108]
The behavior has been resolved with the latest Receiver for Windows versions.