Unless

- you're using any of the "toggle" actions from the Virtual Keyboard, 
which will keep pressing a modifier key until you execute them again

- or use any of the Virtual Mouse’s "toggle button"-like actions, 
which will keep pressing a mouse button until you execute them again 
(and OS X will for the most part ignore you keyboard events as long 
as i.e. the left mouse button is pressed)

- or you use AppleScript & "System Events" to post a keystroke

every keystroke actor in the BCK will also always send a release event.

If you want me to look at your Behaviour and/or actions, send me an 
action pack and/or Behaviour I should look at to info@(the domain of 
this website). Instructions on how to reproduce your observations 
would also be welcome, so that I can get an idea of what exactly to 
look at.

Best regards, 
Felix