The following can be observed in many edits done in Potlatch 2, especially in simple mode.
Nodes as one in two. Often a way will be drawn to intersect another not at an existing node but right next to it. It seems that PL2 has a problem with the minimum distance at which it assumes the user means to connect two nodes, and no obvious indication after-the-fact that something is wrong (since, unlike JOSM, nodes only appear when you mouse over a way). This causes not only redundant nodes but bad errors. For example, where a dual carriageway ends, there may be a tiny bit of one-way single carriageway, due to the node at the end of one direction being different from the one where the other direction and the single-carriageway continuation join. Hence the road is not routable in one direction.
Too many tags. This may be a matter of personal taste, but what's the point of adding oneway=no, bicycle=yes, foot=yes, cycleway=no, access=yes, motor_vehicle=yes to every street? (And sometimes people will think they have to fill in every value despite not knowing for sure what they are, leading to bicycle=no on major highways where cycling is actually legally allowed.) Related is the misuse of the designation tag, which, according to the wiki, is only for certain types of object in the UK, but has been incorrectly used for what belongs in name, ref, note, or other tags.
Use of separators other than semicolons. The accepted way of tagging an overlap of two routes is with a semicolon; e.g. ref=I 80;I 90 for the long segment of toll road that carries Interstates 80 and 90 together. But PL2 displays this as red with a warning icon - "This tag contains more than one value - please check" - leading some mappers to "correct" the tag with a slash, dash, comma, or by deleting the second route.