私は奇妙なことに気づきました。Firefox を使用している場合、これは正常に動作します。
require "selenium-webdriver"
# => true 
driver = Selenium::WebDriver.for :firefox
# => #<Selenium::WebDriver::Driver:0x..fc3c6738535bdb898 browser=:firefox> 
driver.execute_script "window.alert('text');"
# => "text" 
アラートポップアップが開きます。
Chrome で同じことを行うと、アラート ポップアップが開きますが、例外が発生します。
driver = Selenium::WebDriver.for :chrome
# => #<Selenium::WebDriver::Driver:0x47f3180ec45394e8 browser=:chrome> 
driver.execute_script "window.alert('text');"
Selenium::WebDriver::Error::UnhandledAlertError: Script execution failed. Script: window.alert('text');;
 An open modal dialog blocked the operation
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/remote/response.rb:52:in `assert_ok'
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/remote/response.rb:15:in `initialize'
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/remote/http/common.rb:59:in `new'
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/remote/http/common.rb:59:in `create_response'
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/remote/http/default.rb:64:in `request'
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/remote/http/common.rb:40:in `call'
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/remote/bridge.rb:590:in `raw_execute'
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/remote/bridge.rb:568:in `execute'
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/remote/bridge.rb:314:in `executeScript'
    from /Users/zeljko/.rvm/gems/ruby-1.9.2-p290@gretel/gems/selenium-webdriver-2.19.0/lib/selenium/webdriver/common/driver.rb:209:in `execute_script'
    from (irb):14
    from /Users/zeljko/.rvm/rubies/ruby-1.9.2-p290/bin/irb:16:in `<main>'
ここで何か間違ったことをしていますか?
環境:
- Mac OS X 10.7.3
 - Firefox 10.0.2
 - クローム 17.0.963.56
 - chromedriver_mac_18.0.1022.0
 - ルビー ルビー 1.9.2p290
 - セレン-webdriver 2.19.0