While editing, tools will sometimes stay in their alternative mode that is normally activated by holding down the Command key, even if the key is released. For instance, the Move tool will switch to Path mode, or the Select tools will switch to Subtract mode. The tool must be manually reset to normal mode in the toolbox.
Unfortunately I have not found a way to reproduce this on-demand. I only notice it has happened when the tools start behaving in a different way than I expect.