Only Switzerland, as far as I can find. It has done it more than once.

The night in 2007
Early on 1 March 2007, about 170 Swiss soldiers were on a night training march near the border. It was dark, the border isn’t marked, and they walked more than a kilometre into Liechtenstein before they realised and turned round.
Nobody in Liechtenstein noticed. A government spokeswoman said it had happened before and that nobody really realised. Switzerland told Liechtenstein what had happened, and the soldiers’ commander apologised.
Liechtenstein couldn’t have done much about it anyway. It abolished its army in 1868, because it was unpopular and expensive.
The other times
In 1985, Swiss missile training fire landed in Liechtenstein and started a forest fire. Switzerland blamed the wind at first, then admitted the mistake and paid several million francs. In 1992, Swiss recruits set up an observation post at Triesenberg, which is in Liechtenstein. They found out when locals came over to ask what they were doing.
Not the only accidental invasion
In February 2002, about 20 Royal Marines on an exercise landed on a beach at La Línea, in Spain, when they should have been in Gibraltar. A fisherman told them they were on the wrong beach, and they left after about five minutes. The Ministry of Defence called it regrettable and embarrassing.
If you like this sort of thing, see handling a salmon in suspicious circumstances or countries whose flag has a different back.
Sources: Wikipedia on the 2007 incursion, ABC News, The Local on the 1985 and 1992 incidents and Forces News on La Línea.
| Year | What happened | What came of it |
|---|---|---|
| 2007 | About 170 Swiss soldiers got lost on a night march and walked over the unmarked border | They turned round; Switzerland apologised |
| 1992 | Swiss recruits set up an observation post at Triesenberg, which is in Liechtenstein | Locals asked what they were doing |
| 1985 | Swiss missile training fire landed in Liechtenstein and started a forest fire | Switzerland admitted it and paid several million francs |
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They can get invaded by any neighboring country simply forgetting to set their parking brakes.
Italy invading San Marino
Italy invading San Marino (previously forgot to include link)
https://www.sanmarinortv.sm/news/attualita-c4/mezzi-esercito-italiano-entrano-errore-san-marino-a79087