Interesting site.
For diacritics lovers, there always is http://www.flickr.com/groups/fancy-diacritics/ too.

Best practices for designing diacritics could be found here: http://diacritics.typo.cz/

(it seems that diacritics are a big issue in Czech Republic ;-)

That was one of the gripes on the site: Miroslav SATAN v Miroslav ŠATAN for example.
It is a lazy convention rather than a good standard?

Perhaps there are some examples in the Flickr group that Riccardo posted.

Is a missing diacritic considered to be a spelling mistake?

It depends on a lot of factors. And, as Nick pointed out, conventions.
For example, in Italian all caps settings, was/is customary to replace accents with apostrophes, but never to omit them. This is eased by the fact that accents happen only at the end of words and that there are several common words distinguished only by one accent. One glaring example is the difference between "e", which means "and", and "è", which means "is".

First ones I found:
http://upload.wikimedia.org/wikipedia/it/1/17/Vietato_sputare.png
http://www.centromanichini.com/manichini_foto/imgp/2702_targa_latta_viet...

Also, compare these two:
http://www.mondodiholden.altervista.org/userfiles/images/liberta%281%29.jpg
http://www.omelie.org/catechesi2007/imageavv2.jpg

Following a link on that site, I see that the Ogonek is not the only complicated accent in Polish!

Even the kreska - the one that looks like an acute accent - turns out to be tricky.

Thanks for the link, Riccardo.
I’d like to add that with Fancy Diacritics, we are looking to collect examples that are unconventional, in one way or another – including solutions that are playful, creative, dated, alternative, rare, only useful in all-caps etc. pp. Some of them are very clever and beautiful, others unfortunate or even atrocious. Those who are looking for a guide how to design proper (text face) diacritics will find better advice on the sites mentioned in the links section.

Nick, these two are pretty small:
Ä, Ü

Jan, that's the dominator's Czech national hockey sweater I showed above.
Here's another, from 1998 (I'm assuming that's the Nike swoosh, not an accent :-)

I'm no expert on this kind of thing, but this is pretty high profile, arguably the greatest Czech hockey player, playing for the national team.

>For instance in many graphic genres it is customary to omit accents on capitals

I've been watching this change here in Quebec over the past few years. Twenty years ago, omitting the capital accents was considered normal, but as computer technology made the lapse more a question of lazyness and not technical expediency- using accents with capitals has become more and more de rigueur.

I know, it is not Hašek, but Jágr, nevermind – it could be tolerable in these special cases – it is not a standard, because they re not just accents, but another letters, so the trouble is, that you don't know, how to read it, if you haven't knewn it before.

Now I really must say like a certain swordsman drifter: "Oro?"