Errors in a double-ended motion estimation system are detected. An image space is divided into blocks, and a motion vector is derived for each block. The space is also divided into indication blocks. Each motion vector is used to identify one or more indication blocks, in which a value is modified. In an example, a backward component of the motion vector is used to project a source flag into one indication block, and a forward component of the motion vector is used to project a destination flag into another indication block. After processing each motion vector, an indication block with no flags set is indicative of pixel data that has not been tracked, and therefore indicative of a motion estimation error. Remedial action may then be taken.