I heard the Abrams was vulnerable to short range anti tank rifle to the rear and top. Several others have been disabled by IEDs and anti tank mines.
Challenger 2 has been penetrated on three occasions, although one of these was by another Challenger 2.
quotes a British MoD spokesman as saying Challenger 2 was:
Well armoured but in an operational theatre it's not the case that you can have absolute protection. This was not in any way new technology - the device involved was the same type of shaped charge that we have seen used very regularly. No-one has ever said Challenger tanks are impenetrable. We have always said that a big enough bomb will defeat any armour and any vehicle.
To help prevent incidents of this nature happening again, Challenger 2s have been upgraded with a new passive armour package. When deployed on operations the Challenger 2 is now normally upgraded to TES (Theatre Entry Standard), this includes a number of modifications including armour and weapon system upgrades.
Even a Sherman would be able to take on one of those Iraqi T-72's.
I heard the Shermans performed poorly against the T34s in Korea!