A spread with single page inherits guides from both pages of the master spread it is based on.
What it looks like:
Steps to reproduce:
It also does happen if we change the size of a document in a New Document dialog. But not regularly and not with all the presets and sometimes only if we change width.
Once document caught this bug nothing helps to eliminate it: re-save, export to IDML, recreation of all guides, I tried all of it. Guides from both sides of master spread continue to show up.
Checked in InDesign 7.5.3 and 8.0 on Windows 7 x64 and InDesign 7.5.3 on OSX 10.7.4.