I was just checking up on Malcolm in the Middle related wikipedia articles (as I do...) and I can across this latest edition to to the "Hal" page.

Throughout the series there have been various occurrences to suggest that Hal is mentaly ill. This is confirmed in the episode "[[Chad's Sleepover]]" when Hal sympathizes with Dewey's friend Chad, who has mental issues, and Hal reveals that he spends his free time filling in missing letters, circles, and percent signs into an encyclopedia.
Link to page.

On first reading I was like - rubbish and was about to undo the new addition, but the person sort of had a point about Chad's Sleepover. But various occasions throughout the series? Has this ever dawned on anyone else? Does this have any basis? Filling in circles could be considered OCD (obsessive compulsive disorder) but as a one off I think saying he is mentally ill is a little extreme. However if there are other examples then....I dunno...?