I'm using SIMBL in one of my product and run into the problem that the LaunchAgent was not launched on some customer systems. It could be that the problem started with Mac OS X 10.8.3.
net.culater.SIMBL.Agent.plist:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>net.culater.SIMBL.Agent</string>
<key>Program</key>
<string>/Library/ScriptingAdditions/SIMBL.osax/Contents/Resources/SIMBL Agent.app/Contents/MacOS/SIMBL Agent</string>
<key>RunAtLoad</key>
<false/>
<key>LimitLoadToSessionType</key>
<string>Aqua</string>
<key>OnDemand</key>
<false/>
</dict>
</plist>
I was able to fix the problem by adding:
<key>KeepAlive</key>
<true/>
Still I'm interested if this issue is because of changes with Mac OS X 10.8.3 or something else.