Method call

Question

I have a library called lib1 and I am calling a method in this library called "myMethod()". So in the main program I have
lib1 lib = new lib1();
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lib.myMethod (a,b,m,n);
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This is OK surely. However, after the call gets sent to myMethod and returns instead of passing to the next instruction it calls "lib.myMethod(a,b,m,n)" and then I get a "ArrayOutOf Range exception.

What is happeningg??