The newbie again...
Are there any common reasons for an interrupt (ext. interrupts 0 and 1) to fire without the corresponding interrupt request flag being set? I have absolutely no clue (except direct jumps into the Interrupt Vector table which I don't have) how this can happen.
I have about 600 lines of code (so the error isn't really easy to find) and as soon as the SEI command enables interrupts (the interrupts 1 and 0 are previously enabled), the interrupt vectors are being repeatedly jumped to. None of the two interrupt request flags is set through the whole process though. I ran the thing step by step in the AVR Studio 4.13 simulator and found it out there.
I double checked the PORTD, DDRD and PIND registers (interrupts 1 and 0 are on pins of this port) and tried different settings (output, pullup, whatever) there but no change.
...why? ISRs should only fire when the flag is set!