I'm talking about the Selection Locks at the bottom right of the screen, in particular the one called "Drag Elements on Selection".
In the words of Captain BIMCad, it is the "Little Button of Evil". I am glad that someone else considers this little icon to be the bane of Revit Users lives.
It is astonishing how many Revit users do not have this set correctly - unfortunately the default out of the box Revit setting is wrong. Just Plain Wrong! Downright Evil!
The correct setting is for the little evil icon to have a red cross on it so that users cannot select and move an element with one mouse movement - something that is all too easy when trying to make a multiple selection by dragging the mouse. When "Drag Elements on Selection" is disabled, the user would have to select an element, let go of the mouse button then click and move again - that is a more definite action that is unlikely to be done by mistake.
To solve this once and for all, you need to have a BIM manager that sets this properly in your Revit.ini file, and an IT team who understand how to roll out Revit.ini files to all users (that is a challenge in itself). The fall back position is to manually check (and change) the setting on every computer in your office, for every user profile, including whenever a new person joins the company.
The other selection locks are extremely useful, but cause endless confusion because most users just don't see what is right in front of them. The same settings are also available from a drop-down menu below the selection icon at the far left of the ribbon.
It is really important that everyone makes the best use of these, so that you can avoid accidentally selecting things like underlays, linked files and pinned elements. Obviously you just disengage the locks when you do need to select any of those elements. and remember to re-engage the locks afterwards. But never, ever disengage the 'Drag Elements on Selection' lock - it should always have a red cross on the evil icon, and not be ticked/checked in the drop-down menu list.