Do you know the difference between NTSC and PAL Atari games?   From the 2600 FAQ and console compatability FAQ:

NTSC (National Television Standards Committee), PAL (Phase Alternating Lines) and SECAM (SEquentiel Couleur Avec Memoire) are different worldwide, generally incompatible television standards. Some short, not completely accurate information follows.

what                where                      freq/frames/scan lines
----                   -----                            ----------------------
NTSC             US/Japan                       60hz/30/525
PAL                Europe                           50hz/25/625
SECAM         France & many others  50hz/25/625

So how does this affect Atari games?

Each cartridge tells the 2600 when to generate TV line sync pulses. This means that using an NTSC cartridge in a PAL console, or vice versa, will cause synch pulses to be generated at the wrong frequency. The net effect of this is that some TVs will "roll" the picture, and may need the vertical hold control to be adjusted. Some TVs will handle the incorrect synch frequency transparently though.