As I wrote a few years ago, 

the most difficult and important part of an appellate attorney's job is identifying possible issues. An appellate record is like a puzzle that the attorney must solve to discover what issues are present and worth raising. An issue not raised is not likely to result in reversal. That should be enough incentive to scour the record for possibly meritorious issues. If not, the possibility that the court may reverse a conviction on an issue which you failed to find or declined to raise, but was raised by a client in a supplemental brief, should provide motivation.

The decision of the Appellate Division, Fourth Department, in People v Kirk (2012 NY Slip Op 04461 [4th Dept 6/8/12]) is another reminder of how unsettling it is for appellate counsel to have an appellate court grant relief on a ground not raised by counsel, but in the appellant's pro se supplemental brief:

Defendant's remaining contentions are raised in his pro se supplemental brief. Although defendant's contention that the indictment was duplicitous on its face is not preserved for our review (see People v Becoats, 17 NY3d 643, 650-651), we nevertheless exercise our power to review it as a matter of discretion in the interest of justice (see People v Bracewell, 34 AD3d 1197, 1198). .. We conclude.. that counts 2-7, 19-20, 22, 24 and 26-27 "were rendered duplicitous by the trial evidence tending to establish the commission of [multiple] criminal acts during the time period[s] specified [with respect to those counts]" (People v Bennett, 52 AD3d 1185, 1186, lv denied 11 NY3d 734; see generally People v Keindl, 68 NY2d 410, 417-418, rearg denied 69 NY2d 823). We therefore further modify the judgment by reversing those parts convicting defendant of criminal sexual act in the first degree under counts 2-4, sexual abuse in the first degree under counts 5-6, 19-20, 22 and 24 and sexual abuse in the second degree under counts 7 and 26-27 of the indictment and dismissing those counts without prejudice to the People to re-present any appropriate charges under those counts of the indictment to another grand jury (see Bennett, 52 AD3d at 1186; Bracewell, 34 AD3d at 1198-1199). 

The danger in failing to raise non-frivolous, but unpreserved error it that you might one day have to write to your client about the issue he won that you did not raise.