Created attachment 97540 [details]
[ ^ video of the bug ]

Hi,

Here is a bug very damaging : tiles crashes on the canvas and content of the layer is lost. One of the most frustrating bug of 2016 while drawing/painting with 2.9.11. It's a hard bug to find or trigger, and also very hard to propose a way to reproduce it. I try to hunt this one since january because I experience it on weekly basis. Today I started to record a video, and the bug happens. ( a 5 minute video in attachment ). So, I'm filling this bug with this video because maybe the video will give you hint of what really happen, and maybe how to prevent it. I totally understand if this bug keep a long time in unconfirmed, or in a 'wait' status. Thank you for your time studying this one.

Comment about the video ( in attachment ):
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(Note: I'm using 'Temporary save tweaks to preset' in the brush editor)
* (3:00min) I switch from a a pixelbrush engine preset ( temporary saved with 'Eraser' blending mode ) to a Deform engine preset, to tweak proportion. The Deform-engine preset use the 'Copy' blending mode. 
* (3:32min) I press 'M' I enter in mirror view mode.
* (3:34min) I switch back to my pixelbrush engine preset, and press 'E' because this one was in erase mode. Note how the blending mode is not restored to 'Normal' but change to 'Copy' ( *BUG*) of course, I start to notice it while painting. I try to press 'E' but the eraser switch is blocked. 
* (3:54min) I manually go to the blending mode drop-down list in the toolbar, to switch back my tool to 'Normal' blending mode. I paint, but the result is still a 'Copy' blending mode. 
* (4:12min) I'm zooming in and out, to try to solve this issue by refreshing the canvas zoom. No success. 
* (4:17min) I'll switch visibility of layer on/off ; it will trigger the major bug here. When visibility is back , half of the tiles crashed ; the drawing is good to go to trash.
* (4:23min and after) A series of attempt to recover the drawing, duplicating, putting the layer inside a group, etc... No : the datas are lost.