Bug
May need to repeat steps 3 and 4 multiple times for the Exception to happen.
No errors in developer console.
Got the following errors:
Uncaught DOMException: Failed to execute 'setEnd' on 'Range': The offset 6 is larger than the node's length (5).
at window.CKEDITOR.window.CKEDITOR.dom.CKEDITOR.dom.range.getClientRects (https://sdk.ckeditor.com/vendor/ckeditor/4.10.0/ckeditor.js:187:3)
at getViewPosition (https://sdk.ckeditor.com/vendor/ckeditor/4.10.0/plugins/autocomplete/plugin.js?t=I6D6:14:470)
at g.updatePosition (https://sdk.ckeditor.com/vendor/ckeditor/4.10.0/plugins/autocomplete/plugin.js?t=I6D6:18:76)
at f.viewRepositionListener (https://sdk.ckeditor.com/vendor/ckeditor/4.10.0/plugins/autocomplete/plugin.js?t=I6D6:11:217)
at f.
at a.h (https://sdk.ckeditor.com/vendor/ckeditor/4.10.0/ckeditor.js:10:70)
at a.
at a.window.CKEDITOR.window.CKEDITOR.dom.CKEDITOR.editor.CKEDITOR.editor.fire (https://sdk.ckeditor.com/vendor/ckeditor/4.10.0/ckeditor.js:13:97)
at CKEDITOR.plugins.undo.UndoManager.type (https://sdk.ckeditor.com/vendor/ckeditor/4.10.0/ckeditor.js:1079:373)
at CKEDITOR.plugins.undo.NativeEditingHandler.onKeyup (https://sdk.ckeditor.com/vendor/ckeditor/4.10.0/ckeditor.js:1090:121)
As described it does not reproduce all the time, for me I was able to repro it when I had the inserted mention phrase in the middle of the plain text.
Looks like an issue with our webkit filling characters.
I'm also able to reproduce the issue. It seems like it occurs more often when you start deleting adjacent text node.

I can also reproduce this issue. I also faced another one which I randomly get while trying to reproduce this one. Not sure if it is related:
plugin.js?t=I6D6:7 Uncaught TypeError: Cannot read property 'element' of null
at Object.CKEDITOR.plugins.textMatch.getRangeInText (plugin.js?t=I6D6:7)
at Object.CKEDITOR.plugins.textMatch.match (plugin.js?t=I6D6:6)
at b.callback (plugin.js?t=I6D6:6)
at b.<anonymous> (plugin.js?t=I6D6:5)
at c (ckeditor.js:27)
@pupi1985 this issue is related to #2373
@dennisjsq @pupi1985 thanks for reporting the issue, but I'm closing it as duplicate of #2194. Further discussion should be hold there or in PR #2370 which holds the fix.