The general rule that character evidence is inadmissible in criminal proceedings is always subject to exceptions.  One of those exceptions is when the prosecution "opens the door" by introducing evidence of the victim's virtuous life during the penalty phase.  But in a murder trial, is evidence that the victim possessed child pornography enough to sully his good character?  And if defense counsel fails to introduce that evidence, is it reversible error under Strickland v. Washington? 

Yes, says the Supreme Court of Missouri in the recent case of Gill v. State:

When the State introduced evidence regarding the victim's good character, Gill's counsel should have presented rebuttal evidence. However, his counsel failed to present rebuttal character evidence because they failed to discover it. They saw the names of the sexually explicit files on the report but were not diligent in investigating further. Defense counsel testified at the Rule 29.15 hearing that after more carefully reviewing the report, they noticed entries that should have alerted them to the presence of pornography on the computer. They had the opportunity to discover the child pornography, bestiality content, and instant message conversations on the victim's computer but neglected to do so.

By failing to discover those files on the victim's computer, Gill's counsel's performance was deficient.