This bug is already fixed in the SVN version.

Reopen the report since the problem is reproducable for me with KSpread from
1.6-branch compiled 5 minutes ago. Also I changed the severity to grave since
wrong formula-results are one of the worst cases.

Reproduce;
1. Fill cells A1="1", A2="10", A3="100", A4="=A1+A2"
2. Change A4 to "=A2+A3"
3. Undo
4. A4 has the old calc-result and even F9 does not update the wrong value. Only
way to get the correct values is to close the document and reopen it.

Proposed fix;
Index: manipulator_data.cc
===================================================================
--- manipulator_data.cc (Revision 574496)
+++ manipulator_data.cc (Arbeitskopie)
@@ -71,6 +71,13 @@
         {
           Cell *cell = m_sheet->nonDefaultCell (col, row);
           cell->setCellValue (val, fmtType, text);
+          if (cell->isFormula()) {
+            cell->setCalcDirtyFlag();
+            //cell->calc();
+            //m_sheet->valueChanged(cell);
+            //m_sheet->recalc(true);
+            //m_sheet->updateCell(cell,col,row);
+          }
         }
       }
     }

The problem with the fix is, that even if the cell's dirty-flag is set and even
if calc() got called, the formula does not got updated whyever :-(

Added Tomas to the cc-list since that way it's easier for him to know, that I
reopened the bug + confirmed the bug (maybe a regression?)

This with 1.5.x or trunk (Qt4) ?

SVN commit 598528 by nikolaus:

Fix "undo/redo apparently is not recalculating formulas"

BUG: 128383


 M  +4 -0      branches/koffice/1.6/koffice/kspread/kspread_cell.cc  
 M  +4 -0      trunk/koffice/kspread/Cell.cpp  


--- branches/koffice/1.6/koffice/kspread/kspread_cell.cc #598527:598528
@@ -516,7 +516,11 @@
 void Cell::setCellValue (const Value &v, FormatType fmtType, const QString &txt)
 {
     if ( !txt.isNull() )
+    {
         d->strText = txt;
+        if ( isFormula() )
+            makeFormula();
+    }
     else if ( !isFormula() )
         d->strText = sheet()->doc()->converter()->asString (v).asString();
     if (fmtType != No_format)
--- trunk/koffice/kspread/Cell.cpp #598527:598528
@@ -329,7 +329,11 @@
 void Cell::setCellValue (const Value &value, FormatType fmtType, const QString &txt)
 {
     if ( !txt.isNull() )
+    {
         d->strText = txt;
+        if ( isFormula() )
+            makeFormula();
+    }
     else if ( !isFormula() )
         d->strText = sheet()->doc()->converter()->asString( value ).asString();
     if ( fmtType != No_format )