Version:           4.2 (using Devel)
Compiler:          gcc version 4.3.2 
OS:                Linux
Installed from:    Compiled sources

It's happen in any command for example: kcalc

write kcalc and press enter, it may work..
try again and again...

it will not always work...

The problem is here:
void Interface::runDefaultResultItem()
{
    if (m_queryRunning) {
        m_delayedRun = true;
    } else {
        run(m_resultsScene->defaultResultItem());
    }
}

If I just change it to:
void Interface::runDefaultResultItem()
{
   run(m_resultsScene->defaultResultItem());
}

It work, however it will open the first searched application even if you type wrong for example: kcalca (I guess it because didn't start the new search and open the first application from the previous search kcalc)