It's not a bug. You have a setting, formula or link that causes excel to think you made a change. Try Tools|Options|Calculation uncheck Update remote references and Save external link values (just for testing, don't keep it this way if you link sheets).
Also test by closing sheets, set recalc to manual, then open and close your spreadsheet. If this avoids the problem you know its a formula in the sheet. Also check if there are macros. Can't say exactly why but macros trigger this behavior a lot.