Common yeast can survive Martian conditions

How likely is it that we have already accidentally "contaminated" other planets or moons, despite procedures to prevent this?

It seems unlikely to be possible to completely prevent all lifeforms from hitching a ride

Tardigrades (crash) landed on the moon: https://en.wikipedia.org/wiki/Beresheet

It’s pretty likely or at least plausible.

https://www.liebertpub.com/doi/10.1089/ast.2011.0738

I worked with these organisms from the space station. Their resistance to things like hydrogen peroxide (what we wash space probes with) is incredible.