just a small list of bugs i`ve found sofar, most of them are probably already mentioned in other threads some may not.
screen refresh bug (or atleast thats how i´d call it): occasionaly happens if i for example undo a stroke, the stroke is undone but he´s still visible and only disappears if i either overpaint the whole area of the stroke with a new stroke or for example push the eye icon on a layer (a layer like the main canvas layer will refresh the whole picture while a layer with only a few small strokes will only refresh a small part of the picture). while playing around i observed that this bug doesn´t occur anymore if i turn all layers to invisible except the one i´m currently working on, hope this helps somehow fixing that bug since its one of the most anyoing i´ve found sofar.
lasso tool bug: painter crashs without any warning or error message while using the lasso tool. i couldn´t figure what exactly needs to be done to make painter crash that way though it happens way more frequently when i´m playing around with my wacom pen (intuos 3), i think i couldn´t reproduce it with the mouse.
horizontal fading stroke: brush stroke ends with a horizontal fading line even if the whole stroke was vertically, seems to appear very random, sometimes 50 times a hour sometimes not once during a whole week. could be a problem with the tablet, maybe a disturbing signal caused by a too close to the tabled placed cellphone or something although i´ve never experienced this bug in photoshop, only with painter.
layer bug: the two layer options protect transperancy and absorb background colour are active even if the hook isn´t checked. seems to happen if i previously worked with those options and closed the programm without unchecking those boxes. can be turned off by checking and unchecking the box again. simular problems seem to appear with the eye icon of each layer (not sure how this gets triggered).
brush designer: by opening the brush designer painter turns the workingspace into a chaos (seems to open like a seperate application), thats why i avoid using the brush designer and work via window->brushtool settings, unfortunatly features like the randomizer don´t get involved in the working process this way.
preasure control: some brushes seem to jump there opacity from pretty soft to very hard in a way that makes it difficult to control with pen pressure, even by chainging the pressure presets. i think a good example are the oil brushs. i´m not sure if this is a bug or feature (i´ve rarely used any oil colour myself yet), but a little fine tuning might be a improvement here.
thats all for now. hope this helps somehow. if anyone found a new bug or wants to add something to those mentioned above feel free to do so.