Typing '/xxx/' automatically transforms into 'xxx' in italic. This also happens in Calc and Impress.
This is not a bug. This feature is controlled in Tools ▸ AutoCorrect ▸ AutoCorrect Options..., at Options tab, as [] [] Automatic *bold*, /italic/, -strikeout- and _underline_ option.
The convenience feature can be disabled or you just undo the last operation. It's like converting a http:// into a proper hyperlink making it uneditable without the dialog.