I am having trouble reproducing this. This could be a language specific problem. What language did you see this in? Could you provide a short code sample where I can reproduce this? Does this happen in a default configuration also?
When you scroll down and accept "ToBin" as an argument, how are you doing that, there are numerous ways to accept a completion (TAB, ENTER, typing open paren, double-clicking on it with the mouse, ...). Also, what sort of a completion is it at that time? (Current Context, Symbol, Word completion)? Is function-argument help active at the time?