Side effects when passing by reference
Bugs result when instances of parameters unexpectedly change their value. To put it at its simplest, parameters changing their value are a side effect, and side effects trip up programmers. If I call the function
Dim a, b a = f(b)
it is immediately apparent to me that a is likely to change, and probably b will not. But I cannot guarantee that without looking at the source code for the function f.
It is particularly unfortunate that the default parameter-passing convention is ByRef, because it means that you have to do extra typing to avoid ByRef parameters. Because of backward compatibility, I don't think there's any chance of this being changed. However, it has been suggested by the Visual Basic team that they could fill in the ByRefs automatically on listback in the editor so that people can see exactly what's happening. The problem is that this would alter all old code as it is brought into the editor and cause problems with source code control.