Gmail has recently added a new tab to Gmail settings, the Labs tab where us Gmail users can offer feedback on proposed Gmail features.
In Introducing Gmail Labs the Gmail Team says:
The idea behind Labs is that any engineer can go to lunch, come up with a cool idea, code it up, and ship it as a Labs feature. To tens of millions of users. No design reviews, no product analysis, and to be honest, not that much testing. Some of the Labs features will occasionally break.
If you are trying out any experimental Gmail Labs features and are having trouble read Disable Labs . Happy Labbing!