This article is from the comp.sys.acorn FAQ, by Paul Vigay with numerous contributions by others.
This is due to a bug in the RISC OS 3.5 Wimp module. Applications that have outline fonts in their icons and a validation string of R5 or R6 (slabbed icon) will trigger this bug and cause the desktop to revert to the system font. There are two solutions to this problem depending on your level of computer literacy.
The easiest solution, for people who are very shy of template editors, is to complain to the author(s) of the application about this problem and get them to fix it. If you are not shy of template editors you could also go in and edit the applications templates so that none of the slabbed icons have outline fonts in them.