It doesn't exactly help when sometimes you have Dupont and Dupond in charge of testing, remember the Óscar Pereiro in the restaurant story? He availed himself for testing at his house from 7 to 8 am, they showed up from 7 to 8 pm and insisted on testing him, though he was at a restaurant with friends and under no obligation to be anywhere else. Then there wasn't enough room in the restaurant toilets to perform the tests so they had to do it in the hallway. Which, considering some of the rules regarding the tests (for example, that the subject must be stripped to the ankles), could be considered not just an affront to Pereiro's rights, but also could open them up to being sued by the restaurant, as they have every right to be upset about this kind of commotion taking place in a place they're trying to serve food. Plus, of course, given all that went on with the test, the risk of contamination of the sample being high, it would have been nigh on impossible to actually convict Pereiro of any doping offence from that test.
So the testers are fallible, make scheduling errors and do stupid things and make excuses for them too.
I'm sure some people will see this unfavourably for Cavendish, and likely unfairly as after all, there is no sanction required, requested or necessary for a first strike, as the riders are only human and sometimes the requirements of the whereabouts system are difficult to adhere to, even for the most basic of reasons, say you got stuck in traffic and couldn't make it to the place you said you'd be on time, or because you forgot about a prior engagement, or because a meeting you were in overran. At the end of the day, as long as this is just in the cycling press, it's not going to change anybody's opinion of the guy, since it's a complete non-story.