A judge hears a pre-trial motion to suppress and denies same.

During trial he sua sponte revives the motion to suppress and grants same. Directed verdict is then granted.

What rule/case law allows him to do this?

I think that would be the rule of might makes right. I recall something similar happened when Judge Onion took a similar approach in the Kay Bailey Hutchinson case. Something about she had a privacy interest in the documents of a state agency?

It would be ideal to have a statute that required suppression motions and rulings to take place before trial.

"A supression ruling is not a final judgment. * * * A trial court may reconsider its pretrial suppression order." 11 S.W.3d at 322. Its the pits when it happens, but must have been a close question all along. This is one reason why I think Mercado is unfair. The defendant gets a second opportunity to exclude the evidence. No estoppel and apparently no limitation on raising a new legal theory. Most judges do not change their mind very often, but I guess you can lay behind the log or treat the suppression motion as a trial balloon if you are really gutsy.

Thanks Martin, I didn't think it was but, of course, the judge did. And his vote counts more.