Tooltip entry is erased if FMP doesn't like your syntax
FMP 12.0v1 (also 11v4)
Operating system version
OS X 10.6.8
Description of the issue
In some areas of the application, such as when entering tooltip text or a script variable without opening the calculation dialog, FileMaker will try to determine whether your entry is a parsable expression or just a text string (in the latter case, it adds quotation marks and otherwise accepts your entry). If your entry resembles an expression (i.e. contains unquoted parentheses?) but the parsing fails, it rejects your entry and essentially tells you to fix it. (Other areas such as open-ended script step parameters won't auto-add quotation marks for you, but they'll still alert you to a syntax error and give you a chance to fix your entry.)
However, when a tooltip entry is rejected, you get a "The specified table cannot be found" message (twice in succession, as of FMP12 on OS X), and your tooltip entry is reverted back to whatever it was before your rejected entry, giving you no chance to correct what you previously typed. (FMP11 exhibited similar unfriendly entry-reverting for tooltips, but it only showed one rejection message, not two.)
In FMP11 and FMP12, tooltips are the only calculation-outside-the-calculation-dialog area that will revert away your entry; all others let you correct it.
Steps to reproduce the problem
Without using the pencil button to enter the calculation dialog, try to assign a tooltip value of "GetValue ( nosuchfield; 1 )" without the quotation marks (assuming, of course, your file has no field or custom function called "nosuchfield"). After a dialog (two identical dialogs in FMP12 on OSX), your entry will be reverted away (cleared, if you had nothing for a tooltip beforehand).
As in the script variable entry (or, say, any number of parameters used in script steps), you should be told (once) that the table couldn't be found, and you should then be given a chance to correct your entry.
Your invalid entry disappears, whether you were trying to type a calculation and messed up a field name, or you were typing text that you had good reason to expect FMP would auto-quote for you as usual, but you didn't realize a parenthesis therein would make FMP object.
When entering a tooltip, do so only in the calculation dialog, or else you risk losing what you typed in the event you typed something wrong.