Monster By Mistake was a French-Canadian animated cartoon series that was broadcasted on YTV from 1999 to 2006. The plot of the series is siblings Warren and Tracey finding a spell book, which Tracey uses to accidentally transform Warren into a monster whenever he sneezes. When he sneezes again, he turns back into Warren. Information on the episodes are hard to come by except for a small synopsis on each episode on http://www.catapultproductions.com/Behind/index.html
Only 3 episodes have been uploaded to the internet, with the rest of the 52 episodes being lost. However, a few episodes are available on DVD.
1/29/2016 EDIT: 7 more episodes have been uploaded! The uploader may have more, but I have found the same episodes of higher quality on here: http://www.amebatv.com/series.html?req=17