[curt.clifton]

That's a recurring bug that disappears whenever I try to reproduce it. (Sometimes developers call these Heisen-bugs. Lame quantum physics joke.)

You might try making a new script with the code:

tell application "OmniFocus"
	activate
end tell

There's a thread on the forum about the bug with some possible workarounds. IIRC, merely creating a new script in Script Editor and copying the text of an old, broken script into the new script was sufficient to make the problem disappear. Truly bizarre.