Hello Mr,
I don't recollect,But for the INTx to work the way you like you must configure it for interrupt on transition not the logic level.If you have configured it for the logical level activated interrupt then it will execute only one instruction after RETI and again enter your interrupt routine.I have even used the INTx for single stepping my application from with my application.

Also make sure that your GATEx bit is 0.
Atmel's chips will NOT fail even if you insert it wrongly for about a minute or so.So there is no question of burning a chip by grounding a pin.

S.Sheik mohamed