According to the docs with the Wasia hack, you can't go back to the original firmware if any of the custom functions have been set to non-Canon settings. If your new firmware is the hack the "watchouts" page of the Wasia docs may be useful. This is page 3:
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Watchouts
• Camera may lock up if you attempt to change the language, maybe possible to change the language but
must be done on the original Canon 300D 1.1.1 firmware
• Some features may not work correctly if you are using another language - use English as a workaround
• If you upgrade and some of the things don't seem to work properly, set the settings in the Custom
Function back to the original values (see below) and install the original 1.1.1 firmware, then upgrade to
the new firmware
• Change the SET button to something other than FEC before upgrading to this version
• If MLU does not work, ensure that the camera is in single drive mode
• When using a flash with the MLU enabled, the flash will fire when the mirror lifts and again with the
shutter opens
• Menu items for the MLU timeout will show a few arrows – ignore the arrows
• The quick erase may have altered something with the code used for monitoring the battery, and the low
battery icon may appear earlier than expected
• Numbering of the custom functions are incorrect but the functions themselves are correct
Issues resolved from previous version
• When using MLU and the countdown timer, the 10 second countdown now behaves normally, then
followed by the MLU
• Right button mapping has been removed as it didn’t do anything before
Revertng back to the original version
• Reset all the Custom Function values to the original 300D settings. Note, you cannot use the Reset
option in the menu, it does not affect these values. Below is a list of default binary values from left to right
(the bottom numbers in the CF menu):
0,0,0,0,0,1,0,11,0,0,0,0,0,0,0,0,0
• Flash back to 1.1.1.
Web Resources
• Yahoo group
http://groups.yahoo.com/group/canondigicamhacking
• DPreview Canon 300D forum
http://forums.dpreview.com/forums/forum.asp?forum=1031
• Canon EOS 300D Digital Rebel Tips and Tricks
http://www.bahneman.com …digital-rebel-tricks.html
• Canon 300D Firmware 1.1.1
http://web.canon.jp …ital/E3kr_firmware-e.html
• Canon10D manual (PDF)
http://www.powershot.com …ustomer/pdf/EOS_10D_E.pdf
• Canon 300D manual (PDF)
http://www.canoneos.com …rebel/download/manual.pdf