Shaving off or growing facial hair can cause a similar problem, or adding or removing spectacles, or wearing a hat or an unfamiliar color. Another common hindrance is seeing a person 'out of context', or in a place with which the face-blind person does not associate them. Some face-blind people can not pick their family out of photographs, or recognize them in crowds. The pair does not even need to look alike by normal standards, simply because the face-blind person is unable to use 'normal standards'. They will look alike however in the respects that that particular face-blind person uses for identification.
Most people expect to be recognized within a tenth of a second.
She looked straight at me. This is usually no problem when you have a working automatic face processor in your brain. It will recognize any known face within that time. For a face-blind person however that face processor does not work, and all alternative methods of identifying someone are usually much slower. After recognizing someone you are supposed to greet him or her.