This is a known issue introduced when FileMaker 11 was released.
For More Information see: Annoyance: Expression editor 'knows' only 12 tab positions
This is one of many acknowledged bugs that can be found in the Known Bug List thread here in the Report an Issue section of the forum.
It can also be downloaded as a database file from: https://www.dropbox.com/s/jt09b82i0xijbu3/FMP%20Bugs.zip
Actually this wasn't a bug that was introduced with FileMaker 11, it was an attempt to match the Windows version that has allowed us to lay out calculations and custom functions using unlimited indents (ctrl tabs) since at least FileMaker 9.
We were celebrating the introduction of (alt) tabs introduced with FileMaker 11 in the Mac calculation window until we discovered the problem reported by Steven. Then all our clearly laid out calculations in Windows or those copied from a text editor turned out to be unreadable when viewing them on a Mac.
And we reported this problem immediately with FMP 11.0v1 to FMI, with similar detail to that provided above.
Here we are with FMP 12.0v2 and this problem continues to exist, along with a heap of other bugs that have been brought over from version to version. The report referred to by Phil was made on 16th May 2010!
But, hey ho, who's to care, at least we now have rounded corners on our fields! Far more important than tools to help us with the 70 hours we've spent this week creating one script consisting of thousands of lines of calculations and umpteen custom functions.
The simple answer to most of these issues is that you can't prise money out of people by advertising "hey, we've finally fixed this bug with this release".
At least FileMaker have given us a good tool to do this - the Windows version! (Please, no Mac/PC comments, we've been devoted Mac users since 1985, but horses for courses and this course demands Windows).
I really would love to read a statement from FileMaker officials on this issue.. sad enough, something so undoubtedly useful like a bug tracking database has to be provided by users like PhilModJunk.