Hello
I haven't been able to find this very same bug in the trac or in the sage-level group, even though some related ones have been reported
L = (u + t)/(u - t)
factor( L.substitute(t=sqrt(u)))
yields
(u + 1)/(u - 1)
which is wrong (the example is small enough to be verifiable by hand).
Mathematica, OTOH
l[t_] := (u + t)/(u - t);
Factor[l[Sqrt[u]]]
returns the correct result
(1 + Sqrt[u])/(-1 + Sqrt[u])
If this is a previously unknown bug, I would like to submit it (or if anybody wants to submit it and inform me, they're welcome!)
best
Roberto