When allowing a "title" with an image field, the textfield element allows a maxsize of 500. However, the default column size is VARCHAR(128). If someone types more than 128 characters into the field you end up getting the "an unknown error has occurred" message.
This patch gives a default maxlength of 128.
On a related note, there are variables for the maxlength of the alt and title fields, but nowhere to set them through the UI. Would it be worth it to add that config to the field/instance settings forms?