The following problem with **sledgehammer** and *z3* has been reported by Peter Lammich (isabelle-users 05-Apr-2020 00:22) and happens in Isabelle2019 (also in Isabelle2018 or Isabelle2014):

Input:

theory Scratch imports "HOL-Word.Word" begin declare [[smt_trace]] lemma "(max_word::1 word) = 1" sledgehammer [prover = z3] oops end

Output:

... SMT: Invoking SMT solver "z3" ... SMT: Solver: SMT: Result: unsat ((set-logic <null>) (proof (let (($x43 (= (_ bv1 1) max_word$))) (let ((@x94 (monotonicity (rewrite (= (bvneg (_ bv1 1)) (_ bv1 1))) (= (= max_word$ (bvneg (_ bv1 1))) (= max_word$ (_ bv1 1)))))) (let ((@x96 (trans @x94 (rewrite (= (= max_word$ (_ bv1 1)) $x43)) (= (= max_word$ (bvneg (_ bv1 1))) $x43)))) (let ((@x97 (mp (asserted (= max_word$ (bvneg (_ bv1 1)))) @x96 $x43))) (let ((@x352 (trans (monotonicity @x97 (= $x43 (= (_ bv1 1) (_ bv1 1)))) (rewrite (= (= (_ bv1 1) (_ bv1 1)) true)) (= $x43 true)))) (let ((@x356 (trans (monotonicity @x352 (= (not $x43) (not true))) (rewrite (= (not true) false)) (= (not $x43) false)))) (let (($x46 (not $x43))) (let ((@x48 (monotonicity (rewrite (= (= max_word$ (_ bv1 1)) $x43)) (= (not (= max_word$ (_ bv1 1))) $x46)))) (mp (mp (asserted (not (= max_word$ (_ bv1 1)))) @x48 $x46) @x356 false))))))))))) SMT: Time (ms): 11 "z3": A prover error occurred: bad SMT term: _

The final error message stems from the following exception: https://isabelle.sketis.net/repos/isabelle/file/Isabelle2019/src/HOL/Tools/SMT/smtlib_proof.ML#l248