Glenn Morris wrote:

> Philipp wrote:
>
>> You can set the function cell of nil and t using `fset' and friends.
>> But you can't call the `nil' function using (nil) (it does work with
>> (t)).  I think that attempting to set the function cell of nil and t is
>> almost always a bug -- probably the programmer wanted to set a real
>> symbol, but some of the constants got passed.  I propose to signal an
>> error (e.g. `setting-constant') whenever the function cell of nil and t
>> is modified; maybe the same should happen for keywords.
>
> I just did this yesterday...
> See https://debbugs.gnu.org/25110, ba8e883, and 3fd4433.

And now see also ffb1302. :)
Anyway, I only did "nil", since as you say "t" can actually be called as
a function. But you are right that it's probably unintended.