I noticed that sometimes, maybe when moving a line down with alt+page_down, strange characters appear. I use geany 1.33 (built on 2018-04-16 with GTK 3.22.30, GLib 2.56.1) on Lubuntu 18.04.
I managed to take screenshots, with ctrl+z and ctrl+y I could look at the changes: After using ctrl+d to duplicate a line, I tried to move it down. 
After pressing alt+page_down to move it down, it looked like this: 
I pressed alt+page_down multiple times and the characters changed: 
I'm not sure if the steps I described above are correct as I can't remember such a sudden situation exactly.
I do not know how to reproduce this. Also, geany never crashed when it happened.