Your MOT is not a service, just a point in time where the roadworthiness of your car is checked. You can drive out of an MOT station and get stopped by the police for something not being right with your car - you'd have a good argument with the MOT station but you'd still be nicked.
Unfortunatley, because the MOT stresses weak points on your car it isnt abnormal to find something actually fails shortly after.
The rigors of the test can cause a number of problems.