What, all those incoming probes from trojaned PCs looking for new botnet recruits? It may not harm those with a secure setup but such traffic can hardly be considered "harmless" by any means! As for phone-home behaviour, it should be considered at least a privacy risk since if done frequently enough, it can provide a good track of your online presence. Of course, we have the example noted above of Windows "Genuine Advantage" disabling features due to Microsoft's server problems. Getting back to the original topic - pretty much any firewall of consequence has caused problems for at least some users, whether due to "bad coding", conflicts with other "bad" software or drivers or even faulty hardware (problem memory notably). Unfortunately, the requirements for firewalls to have to oversee a wider range of program activities (in order to counter leaktests and malware using similar techniques) increases the chance of things going amiss. Anyone encountering problems should contact the vendor(s) concerned to give them a chance to fix things (and yes, Agnitum do fix things as the Sandbox.sys crash when playing World of Warcraft thread shows) not least since it benefits the user community as a whole - presumably this has been done privately to Agnitum in this case?