Canon i-SENSYS MF729Cx Important Safety Instructions And Installation Manual page 49

When printing on paper that has absorbed moisture
Steam may emit from the paper output area, or water droplets may form on the back side of the operation panel or
around the paper output area. There is nothing unusual about any of these occurrences, which occur when the heat
generated from fixing toner on the paper causes moisture in the paper to evaporate (most likely to occur at low room
temperatures).
LINKS
Registering a Custom Paper Size