MArcomage

http://picasaweb.google.nl/lh/photo/o6ge_N9IAYM_swQilvdM-Q?feat=directlink (before)
http://picasaweb.google.nl/lh/photo/vhcBrMO4ETzWWEntDLkLmQ?feat=directlink (after)

It dealt 38 dmg, while 20 + 17 = 37 (last card was Barbarian) and keywordtrigger is wall -8 for uncommon (can't be it), so I mysteriously dealt 1 bonus damage.

The Barbarian keyword took effect, lowering the wall by an additional amount. In this case, as there was only one wall value remaining, it simply knocked off that last bit of wall.

That would explain it.

I think this should be altered in such a way that it's consistent with how it works with how cards like Warlord work: First lower the Wall, then the attack.
(Alternatively it can be explicitly mentioned @ Game manual that keyword effects always kick in after the effects of the card effects.)

Keyword effects always come at the end, else it might make things complicated.
For example, Acolyte, if Mage effect comes in play first and return some gems cost, you might end up dealing 10 dmg instead of getting the gems boost you needed.
To avoid unnecessary hassles, it seems better to stick with the way it is.

im famous xd

Good point. Take my alternate suggestion.