A common path for new Revit users is to import a dwg file and start tracing a project that got started in AutoCAD. Revit has a number of features that make this pretty easy to do. But just because it was drawn once in cad doesn't make it perfect or correct. I can't remember the last time I used a dwg file that didn't introduce some sort of error into the project I was involved in. What sort of errors am I talking about?
The old they sure look lined up but they aren't problem for one. This one happens alot. Two lines that are on either side of a corridor, forming an opening look lined up but if you look closely you'll find that they are off by a tenth of an inch or less. So when you use the PICK option to place walls you get two segments that are off slightly. The ALIGN command will fix it quick but you'd have to notice it first.
A sure fire hint that there is something wrong is when Revit doesn't offer a snap. If you pick an endpoint of a dwg line segment, move your cursor up to the other end and no SNAP icon is offered, BEWARE! The line segment is not aligned with the previous point.
Another is the line is almost nearly exactly plumb or horizontal, or almost exactly a clean angular value. Such as 89.001 degrees or .003 degrees. Sketching a Revit wall or line over such a dwg source will generate an error message like this one.
This simply means, "Gosh, this line is so close to a nice angle we thought we'd let you know that it doesn't make sense!"
So the next time you start tracing a dwg file to start your Revit model, be a little skeptical and cynical about the dwg source, even if you did the work yourself. Been there, done that!