You may have been told there'd be cake, but the cake is a lie...or is it.
For Portal players we need say no more, but for everyone this is now one of the most well known phrases in gaming. It has come to mean that you are promised a reward that will never appear.
"the cake is a lie" is now referenced in many other places such as the games Dragon Age, Fable and Minecraft as well as the cartoon network.
There is always the possibility that there may be cake though - so just as well keep trying....