Spreadsheet application changes Excel sheet name if the Excel sheet name 
contains a dash (-) character. If you use some kind of scripting where sheet 
names are used, the script becomes unsable.

To repeat execute the following steps:
- Using Microsoft Excel:
 - Create a new Spreadsheet
 - Change Sheet1's name to June-July
 - Save the spreadsheet

- Using OOo Spreadsheet
 - Open the spreadsheet you created with MS Excel
 - Right click on June_July sheet name and select rename
 - The name, as you have noticed has changed from June-July to June_July
 - If you try to change it to it's original name you get an "Invalid sheet name" 
message.