'Hopping' in the IDE?

[Graham Samuel]

I'm seeing something a bit strange while tracing the running of scripts in the IDE (I'm using LC 6.5.1 on a Mac with Mavericks). What I find is that as I watch the activation step through statements, there are lines that invoke other handlers, and as is normal, if I don't choose the 'step over' option, I see control pass to one of these handlers and then eventually return to the main stream of the script. 

So far so good: but I find that sometimes when I step into a statement that doesn't call a subsidiary handler - one example is a statement as simple as

 replace "A" with "B" in myVariable

Then the IDE hops to a subsidiary and shows control passing to the initial 'on' statement of that handler and then moving back to the main script, without the script of the subsidiary being executed. So this change of attention of the IDE seems like a kind of false trail, which is harmless but confusing.

This is very odd but it's also rather elusive. I am still struggling to get a repeatable example, but it looks like a bug from here. Has anyone else seen anything like this?

TIA

Graham