IN LibreOffice Calc, all versions I've tried, I cannot merge two cells if one of them is already the result of a cell merge. So, if I merge two cells let's say A1 and B1, the resulting cell cannot be merged with C1. LibreOffice throws an error saying "Cell merge not possible if cells already merged", This is really annoying and I think LibreOffice is the only open source/free suite that has this problem.(ontherwise, it's a great office app)
I'm agree it's a bit annoying, but after select the range two merge, doing two clicks, helps, the first unmerge, the second merge the selection.
*** This bug has been marked as a duplicate of bug 63766 ***