Web applications and generating alerts due to HTTP requests
One of the consequences and corollaries of never trusting anything you get from the network is that you should think long and hard before you make your web application generate alerts based on anything in incoming HTTP requests. Because outside people can put nearly anything into HTTP requests and because the Internet is very big, it's very likely that sooner or later some joker will submit really crazy HTTP requests will all sorts of bad or malicious content. If you're alerting on this, well, you can wind up with a large pile of alerts (or with an annoying trickle of alerts that numbs you to them and to potential problems).
Since the Internet is very big and much of it doesn't give much of a damn about your complaints, 'alerts' about bad traffic from random bits of the Internet are unlikely to be actionable alerts. You can't get the traffic stopped by its source (although you can waste a lot of time trying) and if your web application is competently coded it shouldn't be vulnerable to these malicious requests anyways. So it's reporting that someone rattled the doorknobs (or tried to kick the door in); well, that happens all the time (ask any sysadmin with an exposed SSH port). It's still potentially useful to feed this information to a trend monitoring system, but 'HTTP request contains bad stuff' should not be an actual alert that goes to humans.
(However, if your web application is only exposed inside what is supposed to be a secured and controlled environment, bad traffic may well be an alert-worthy thing because it's something that's really never supposed to happen.)
A corollary to this is that web frameworks should not default to treating 'HTTP request contains bad stuff' as any sort of serious error that generates an alert. Serious errors are things like 'cannot connect to database' or 'I crashed'; 'HTTP request contains bad stuff' is merely a piece of information. Sadly there are frameworks that get this wrong. And yes, under normal circumstances a framework's defaults should be set for operation on the Internet, not in a captive internal network, because this is the safest and most conservative assumption (for a definition of 'safest' that is 'does not deluge people with pointless alerts').
(This implies that web frameworks should have a notion of different types or priorities of 'errors' and should differentiate what sort of things get what priorities. They should also document this stuff.)