A "pathological" clause is one so poorly drafted that it invites a preliminary dispute about the forum itself, before either side reaches the merits of the underlying claim.
The recurring culprits: naming a non-existent or defunct institution, leaving the seat and venue undistinguished, failing to fix the number of arbitrators, omitting the language of the proceedings in a cross-border contract, and using both "arbitration" and "litigation" language in the same dispute clause without saying which prevails.
Each of these is a five-minute fix at drafting stage and a months-long argument once a dispute has actually arisen. A clause that answers seat, venue, number of arbitrators, appointing mechanism, governing law and language in a few clear lines will outperform a page of aspirational drafting.