TSA has been found to miss stuff quite often.
A DHS Inspector General's findings are that 90% to 95% of dangerous items actually get through screening checkpoints in testing.
TSA fails at least 90% of the time.
Also some reports from the early 2010s found that TSA had high failure rates in the range of 67% to 95% in undercover tests of explosives and weapons.
Although more recent findings are less disclosed by also indicate efforts to improve security with new technology.
TSA also reports that a large number of personal items, like phones and wallets are also left behind at checkpoints monthly, although it is a separate issue from the TSA failing to detect prohibited items.
The airport that has the most strict TSA is San Francisco International Airport.
San Francisco International Airport (SFO) is known for its rigorous security protocols, which are driven by its role as a key international hub.
If the TSA finds a prohibited item in your checked bag, they will typically leave a "hazardous materials notice" in the bag, remove the item, and notify the airline, which may then trash or try to return the item to you at your expense.
If the item is illegal or dangerous, like a firearm, they will contact law enforcement, and you could face arrest and potential fines.