Undo within the processing environment can create problems and has created issues for me. Under heavy cpu load where the screen might not update in real time, holding ctrl+z can lead to erratic behaviour of Undo that can only best be described as it having forgotten the location of the cursor during its history. Random operators or line breaks may be undone leading to a mess of code.

Unsure of how to reliably reproduce, but perhaps this next issue may be a part of the fault:

Double-clicking or highlighting a portion of code and replacing it with a paste from ctrl+v, then Undoing that action will instead altogether delete whatever was there instead of replacing it. Example
int var = 60;
Highlighting 60, ctrl+v "139"
int var = 139
ctrl+z
int var =

Expected behaviour is return to previous, "60".
A second iteration of Undo does restore the previous overwritten value but needing to undo twice for a single action is not expected.

Alternatively, pasting multiple times successively is also completely undone by a single undo

int var = 60;
Highlighting 60, ctrl+v "139"
int var = 139
paste paste paste paste paste paste...
int var = 139139139139139139139
ctrl+z
int var =

I also just now had an issue where after pasting something, my cursor jumped ahead about 8 characters and it was halfway into the name of a variable following what i'd just pasted. Unable to reproduce at this moment but will update if i figure out what it was.