Not all Leopard upgrades have gone as smoothly as hoped, which is not unexpected (and why we always back up before upgrading, right? Right.) and not necessarily cause for enormous alarm. One frequent alert, however, has been the implication of Unsanity's Application Enhancer (APE) framework in some upgrade problems. APE has a long and sometimes controversial history, with some developers swearing BY it (Audio Hijack, for example, uses APE to enable the "Instant Hijack" functionality) and other developers swearing AT it (APE's ability to modify other applications at runtime, necessary to enable some tools, can also make app debugging more difficult).
We relayed an early warning on APE (8:31 pm Friday) as part of Scott's liveblog of his Leopard upgrade, but without confirmation from the developer there wasn't much more to say... until now. Rosyna of Unsanity sent out an urgent email alert to mailing list subscribers (reproduced in whole below) recommending that APE be updated to the current version (2.03) prior to upgrading to Leopard, lest badness ensue. Note that v2.03 of APE still won't actually do anything under Leopard, which knocks out a large slice of utility functionality, but it won't crash either.
In general, if you are doing an upgrade install, I recommend an AppFresh or VersionTracker pass to make sure that all your bits and pieces are as current as they can be. Plus, that backup... don't skip it. Really.