This simply showed up in the middle of a working day. No new objects loaded into editor. Just plain, old fashioned typing.
Thanks for letting the community know... hopefully, this is not happening on a consistent basis, as oftentimes Windows can have issues with memory and, ummm, other things. 
BTW, 16.1 is the latest version of Toad for Oracle, in case you're interested in keeping up to date.
So... this issue is fixed in 16.1 ?
Address FFFFFFFFFF looks a bit "lucky"...
I don't know from your screen shot what went wrong, so I can't say if it's fixed or not in 16.1. It doesn't sound familiar. But it's always a good idea to stay on the latest. You can see a list of bug fixes for 16.1 here.