MobileFrontend creates it's own Userlogin and UserCreate templates. These templates inherits QuickTemplate while core templates inherits BaseTemplate. If another extension (which is included _after_ MobileFrontend in LocalSettings.php) requires functions from BaseTemplate in it's UserLoginFrom or UserCreateForm hook handler (which is legitim, because UserCreateTemplate is the expected class), php ends with a fatal error. MobileFrontend should also inherit BaseTemplate instead of QuickTemplate to prevent these problems.

Encountered with installed MobileFrontend and GoogleLogin extension.