In a similar vein, people are still recommending Cache::Cache or things based on IPC::ShareLite, when BerkeleyDB or Cache::FastMmap would be about ten times as fast. Hopefully my upcoming article based on my talk at ApacheCon will help point people in the right direction on that.
The most recent and most surprising was a big performance bug in Maypole that I discovered while helping Jesse Sheidlower with some performance tuning. People who have used Template Toolkit with mod_perl in a high-performance environment should know that you have to keep the TT object around between requests so that you don't blow the cache and recompile the templates on every hit. Maypole was throwing away the TT object. I gave Jesse a very small patch to fix this and he reported speedups of 250-500% on his application.
What's the lesson in all of this? Probably that being engaged on mailing lists like mod_perl and TT and sites like perlmonks.org has a tangible payoff in terms of knowing what the best practices are. Maybe also that we need to repeat them more often.