Yes. King John of England attempted to take the throne from Richard I while he was on crusade. Richard's delayed return was due to the fact that he had been taken prisoner by Leopold V, Duke of Austria. This from Wikipedia:

When Richard still did not return from the crusade, John began to assert that his brother was dead or otherwise permanently lost.

Yes. King John of England attempted to take the throne from Richard I while he was on crusade. Richard's delayed return was due to the fact that he had been taken prisoner by Leopold V, Duke of Austria, and then handed over to the Holy Roman Emperor Henry VI. John, in the meantime, took advantage of his brother's imprisonment, gathering supporters around him and scheming with Philip II of France. He also

began to assert that his brother was dead or otherwise permanently lost.

Although Richard had named his nephew, Arthur Duke of Brittany, as his heir before leaving for the crusade, Arthur was only a child and John managed to gather around him leading nobles who recognised him as heir. In order to placate John and get his help in raising the ransom money,

Archbishop Walter urged Queen Eleanor of Aquitaine and the regency council to adopt a conciliatory policy towards John....Eleanor and the magnates took Hubert's advice and negotiated a truce with John. He agreed to surrender his castles to his mother and if they were unable to get Richard back, he would become king.

Richard, of course, did eventually return upon payment of a huge ransom. John promptly fled to France but was later forgiven by Richard. In 1196, Richard again named Arthur as his heir but he changed his mind on his deathbed in 1199 and named John instead, probably because he felt Arthur was too young to be king and to command the support needed to hold onto the Angevin empire. Arthur subsequently 'disappeared' (1203), with John being the prime suspect in his nephew's murder.