When editing text in Google Spreadsheets, in the formula bar, or editing the name of the sheet, overwrite is toggled, and pressing the insert key (numberpad or 'regular' insert key, numlock active or not) has no effect on this behavior. I believe I caused this while entering unicode or extended ASCII characters into other webpages. Still unsure of real cause. Google Chrome browser.
How to turn off Insert/Overwrite in Google Spreadsheets
google spreadsheets
When I double click on the cell itself, and then hit the insert button, the overwrite will toggle off for not just that cell but also for the entire application.