Google Chrome version 37.0.2062.94 breaks custom search engines. Yep, that’s right. If you had added custom search engines ( and custom shortcuts ) in your Chrome preferences they will not work in incognito mode with this version of Chrome, at least on Mac OS X. This has been an issue even with this version of chrome was in BETA mode, but now it is also affecting the “stable” version.
It may be worth it to downgrade to an older version of Chrome. If you do this you may get the message about “Your profile is from a newer version of Google Chrome” but that can easily be remedied. ( just search for the message )