I know that this may sound stupid, but yet again I found myself assessing targets (this time, luckily, not levelling for hours) in the wrong equipment.
Therefore, I came to this idea: have a "warning" checkbox for each bought item. When training or fighting without an item where the checkbox is selected, at the end, display a warning saying "You forgot to put something on!", just as the "Something could not be repaired!" is displayed now.