The problem:
In a DataGridView control, after adding a new row, and clicking away, the new row just disappears without any visible error.
The solution:
This is most likely caused by an error being raised due to a data input error. Im my case it was because of a non visible column not allowing nulls.
To find out if this is the cause of yoyr issue, put a break point in the DataError event handler. Even though the error might have beeen written to the row's ErrorText, it is never displayed because the row disappears.