I'm amazed at some of the comments here who seem to say "If you just ignore it, it will get better." I highlighted the ifs above because NOT everyone has the strength to overcome what they perceive as discrimination. Sure we can debate as much as we like whether the different treatment was due to discrimination or not, but let's stick to the real fact here: the treatment is perceived to be different.
When this happens sometimes, we can shrug it off - it's not exactly productive nor courteous to express outrage all the time. And absolutely, it may just be in our imagination, or someone is having a bad day! But when that different treatment happens often enough, no matter how subtle it seems, it can cause pain and affect the person's behaviour and performance.
Not everyone is delusional. Not everyone wants to get the limelight. So if someone is claiming that he/she is being discriminated, listen.