Common issue, and hard to find out, indeed. Often, the error can be in the first tabs, but propagates in the next until it is raised in one of the last tabs. Or the reverse!
I opened one of my sketches with many tabs. I just removed a semi-colon on a line in the fourth tab. I get an error on the second tab, at the start of the last declared class.

A possible solution is to hit Ctrl+T in each tab. If you get a funny formatting, there might be a syntax error in that tab.
Or indeed, compare the files, and double check the syntax of each change.

Thanks for the tip!

If your project becomes big, using a real IDE can be a boost of productivity, even if "real" Java can add a bit of syntax, but overall it is well worth: Eclipse, for example, offers auto-completion, so you have less to type, and can catch syntax errors earlier and in a more precise way. But you might have to start making a file per class, to use visibility modifiers (public, private, ...) and so on.

Thks phi.lho. It kinds of confirms my feeling. 

Some approaches to try:

Thanks Jas, placing the intentional error in different locations and commenting out code makes a lot of sense, if necessary.      

bbcannon said: "I wrote myself a comment as an intentional error so I would remember what I was focusing on when I came back to the project awhile later."

wouldn't that tread be great tutorial or Wiki ?

proposal.

Jas,  I should have gone with option #3 in this case.  Makes sense to create an error that gets past the lexer.  Your other suggestions make a lot of sense also.

You should also try the wonderful Sketch Outline tool for processing.
If you have syntax errors in your sketch it will tell you the line and the tab of the error.
Very useful.

Sorry, what is the link for that?

Thanks a lot!