It seems that the interpreter handles strangely the power series:

a:= series sin(x);
)di type %
   Type of value of %: UnivariatePuiseuxSeries(Expression Integer,x,0)
a=a;
)di type %
   Type of value of %: Equation Any

If I coerce it...
a:=a::UnivariatePuiseuxSeries(Expression Integer,x,0);
)di type %
   Type of value of %: Equation UnivariatePuiseuxSeries(Expression Integer,x,0)
------------------------------------------------
Fatal bug:

a:= series sin(x);
a*1.0;
   >> System error:
   Caught fatal error [memory may be damaged]

protected-symbol-warn called with (NIL)

From trace:
It seems that the interpreter doesn't coerce one args
in Expression (Float):

1<enter valueArgsEqual? : (|UnivariatePuiseuxSeries| (|Expression| (|Integer|)) |x| ((0 . 0) 0 . 1))\(|UnivariatePuiseuxSeries| (|Expression| (|Float|)) |x| ((0 . 0) 0 . 1))

...

And here:

1<enter algEqual : ((0 . 0) 0 . 1)\((0 0 . 0) 0 1 . 0)\(|Expression| (|Float|))
 1<enter compiledLookupCheck : =\((|Boolean|) $ $)\#<vector 08eccc78>
 1>exit  compiledLookupCheck : (#<compiled-function |EXPR;=;2$B;21|> . #<vector 08eccc78>)

after SPADCALL (it's not traced) "=" in EXPR trigger the bug.

algEqual : ((0 . 0) 0 . 1)\((0 0 . 0) 0 1 . 0)\(|Expression| (|Float|))
           ^^^^^^^^^^^^^^^
I think above is not of type EXPR FLOAT.

Cheers.