Was working on a workflow activity today and hit this annoying error:
Cannot use custom activities - failed to load toolbox item
It had the unpleasant effect of crashing Visual Studio (2012). The workaround was to remove the workflow DLL from the GAC (the one that I was working on), then re-open Visual Studio and try again. Worked like a charm after that.
CraigMartin – Edgile, Inc. – http://identitytrench.com