Occurs when AFL engine detect the do-while loop that never ends (the detection is based on number of iterations, AmiBroker simply counts the number of iterations and if it exceeds the threshold limit set in Tools->Preferences->AFL: Endless loop detection threshold - by default 100000 iterations) it displays this message). Example:
i = 0;
do
{
x = i;
}
while( i < 5 ); //
i variable is not incremented, so the loop never ends.