CDec function is now documented in Basic on-line help:
  Converts a string expression or numeric expression to a decimal expression.
  Return value : Decimal number 
This function is known to VB programmers trying to convert VB macros into OOo macros.

Next attachment is a document with macros that demonstrates the following bugs:

Macro howToCrash
this code crashes:
  dim t
  t = CDec("")

Macro strangeConversion
some values have a strange conversion:
  t = CDec("79228162514264400000000000000")
  print t ' gives 62406456049664
Other values give the same result

Macro silentOverflow
  t = CDec("79228162514264340000000000000")
  print t ' gives zero

Macro negativeNumber
this is incorrectly recognized as a negative number:
  t = CDec("  12345   - ")
  print t
  t = CDec("12345-")
  print t
This is not the case for CLng() nor CInt()