I use a rather complex class, built on top of extarticle with twocolumn option. It loads a lot of packages. One of them is floatrow.
Sometimes, I get a very annoying error that can be seen in the screenshot below:

As you can see, the two columns are badly shifted (left column to the right and right column to the left). This only happens when there are figures (not necessarily on the same page of the issue) inside a floatrow environment. Without floatrow, I never got into trouble.
I did not succeed in making a minimal example that reproduces the issue. So I'm looking at a way to track down the issue maybe with some \tracing... commands. How should I proceed?
\tracingall(but this will fill your disk with literally megabytes of log file if you don't agressively trim the example first. Or just the classic "add print statements" debugging system, edit the source and add\show1\show2etc at the places you think it should be executing and move them around until you find where it is making the wrong choice. – David Carlisle Sep 13 '13 at 11:42