The Wikipedia article the previous commenter linked doesn't mention this, and I've looked at a few sources linked from it and they don't mention it either.
They mention that previous Therac machines had "hardware interlocks" to prevent this sort of issue - maybe those are the safety fuses you're both talking about? - but it says they were never included in Therac-25 in the first place, because the software was assumed to be so reliable, not that they were removed because they blew.
Are you sure this is the same incident? Do you have a link to a source about it?
Edit: This article cited in Wikipedia is much more comprehensive and mentions that Therac-20 did have these fuses, in addition to hardware interlocks. But they weren't disabled because the real alarms were drowned out by false alarms - instead they all represented real instances of the issue.
https://web.archive.org/web/20041128024227/http://www.cs.umd...
(Middle column of the 12th page of PDF, numbered page 29 of the publication.)
> The Therac-20 at the University of
Chicago is used to teach students in a
radiation therapy school conducted by
the center. The center’s physicist, Frank
Borger, noticed that whenever a new
class of students started using the Therac-20, fuses and breakers on the machine tripped, shutting down the unit.
These failures, which had been occurring ever since the center had acquired
the machine, might appear three times a
week while new students operated the
machine and then disappear for months.
Borger determined that new students
make lots of different types of mistakes
and use “creative methods of editing”
parameters on the console. Through
experimentation, he found that certain
editing sequences correlated with blown
fuses and determined that the same computer bug (as in the Therac-25 soft-
ware) was responsible. The physicist
notified the FDA, which notified Therac-20 users.*
> The software error is just a nuisance
on the Therac-20 because this machine
has independent hardware protective
circuits for monitoring the electron-beam scanning. The protective circuits
do not allow the beam to turn on, so
there is no danger of radiation exposure
to a patient. While the Therac-20 relies
on mechanical interlocks for monitoring the machine, the Therac-25 relies
largely on software.