Texas Tea TMC

3 common interference possibilities
Rouleaux - look at the reaction under scope, see if you see the coin stack appearance. Resolve IS interference with saline replacement
Nonspecific cold agglutinin - perform mini cold screen   - one group O big I+ adult cell (screen cell works fine), one group O i cell (cord blood), and autocontrol. 2 drops patient plasma to each of those cells.  Read at IS, RT inc and 4C inc. If autocontrol and adult cell are positive, and reactions increase in strength with refrigeration, its usually presumptive nonspecific cold agglutinin. Resolve with prewarming the plasma to reduce interference at IS phase. May not work if cold agg is strong.
Cold preferring IgM antibody like an Anti-M or P that doesnt show in gel but may interfere in tube if the crossmatched unit is antigen positive.  Run a 3 cell tube screen with IS phase and a RT phase to get clean strengths to id the antibody. Your hospital policy should specify whether you need to antigen type units to get real crossmatch compatible, or just crossmatch untyped units until you get a IS XM compatible one. Most hospitals consider the common suspects (M, P, Le) not clinically significant if they are only showing at IS, but if you work at a place that requires IS XM on all rbc orders, and they want it "compatible" you might be stuck antigen typing.
And lastly always a chance the unit or patient ABO result is incorrect, repeat the ABO typing on patient sample and unit.