Slipped

The spy slipped the secret code into the message, without anyone realizing it.

The secret plan was slipped into a confidential memo, without anyone noticing.

He slipped on the wet pavement and sprained his ankle.

He slipped on the banana peel and landed on his backside.

She slipped on the icy sidewalk and slid several feet before coming to a stop.

The building's safety standards have slipped over the years due to lack of maintenance.

The bird slipped out of its cage and flew away.

He slipped a piece of paper into the envelope and sealed it shut.

The mouse slipped through the small crack in the wall.

The spy slipped past the guard and entered the building undetected.

His physical condition slipped after he stopped exercising.