Omar had been doing the Masses and pulley practical but wasn't convinced that the tension in the string was the same at both ends. How can it be when the masses are different. My attempts to explain weren't convincing and efforts to prove it mathematically led nowhere. If you break this down into a three body problem you have to give the rope some mass and then you realise that if the forces were the same at each end it wouldn't accelerate. However if the string is massless the tension at each end will be the same. So Omar was right, the tension isn't exactly the same unless you state the string is massless.