Some apps, for example, open up the freshly installed application after installing. Now everybody propably knows that this can be solved with taskkill by using a .bat. But it doesn't always work out - WPI goes to execute the .bat with all the taskkill commands TOO EARLY, like just a millisecond after the installer tells WPI "I'm finished" - and taskkilling ends up with an error because the app that was supposed to be killed hasn't yet started. Now the simple solution is telling the WPI to wait some seconds before executing the next command (the command that tells to execute the .bat with taskkill commands), so the app that the installer opens up (the one you are going to taskkill), has enough time to actually open. What some people might have tried is %sleep%. Now here's the problem, it doesn't work. The WPI changelog tells us to use %sleep% while the command is actually {SLEEP}. I suggest that you fix the changelog because %sleep% really doesn't work. I created this topic to inform people about this wrong command that WPI changelog tells you to use - and also give you the idea of how this problem of taskkilling ending up with an error can simply be solved. In conclusion, here's the instructions: Line 1. Command is the installer command with silent switches that you normally use. The installer opens the freshly installed app after the instalation has succeeded. Line 2. Use command {SLEEP} 5 to tell WPI to wait 5 seconds before rushing into executing the next command, command three. During these 5 seconds, the freshly just-installed app has enough time to actually launch its process .exe so the next command doesn't end up failing. Line 3. Use the command you normally use to execute the .bat file that has the taskkill command(s). Now that the app had 5 seconds time to launch, taskkilling should end up ok without an error You can use whatever number in place of 5. 5 is actually quite a lot. Usually 2 or 3 is enough time for any app to start its process. 1 is propably not enough. I hope this helps you people.