Explanation: During Caesar's Civil War, the Republican general Pompey escaped to Egypt after a crushing defeat in the hopes of gathering more allies against Caesar. The Egyptian King, Ptolemy XIII, decided instead to kill Pompey, in the hopes of garnering support from the increasingly-powerful Caesar, who seemed poised to win the civil war. When Caesar came seeking Pompey, Ptolemy presented Caesar with Pompey's severed head.
This was a massive miscalculation on several levels.
First, Caesar had a policy of extremely generous mercy towards his opponents in the civil war - he wanted to avoid appearing as a tyrant, and pardoned almost all of his enemies without preconditions, not even a pledge of support. This was an extremely powerful tool to exercise, as it made resisting Caesar less appealing than surrendering - if you lose nothing by surrender, not even your honor, and your cause appears hopeless... why not? By killing Pompey, Caesar was robbed of the opportunity to grant his most powerful enemy - both practically and politically - mercy, which would have been a massive PR coup.
Second, the Romans generally frowned on others dealing out justice (or 'justice') to Roman citizens. Roman citizens acknowledged only the authority of the Republic over them, not foreigners! While some of this posturing is just that - posturing - Roman citizens who had been mistreated by foreign rulers, or even treated fairly-but-harshly, were often the centers of calls for retribution - whether legal or military - upon those who punished them. For Pompey, a member of the Senate and consul, one of the two highest-ranking officials of the Republic, to be murdered by a king he came to with a hand opened in friendship, asking for help? That was deeply offensive to Roman norms - and perhaps more dangerously, Roman pride.
Third, Caesar and Pompey were old allies, going back decades - while some of the rhetoric between them is buried in Roman cultural norms and the forked-tongue statements typical of politicians, it's often considered that there was some amount of genuine goodwill between the two men, even if their ambitions (and egos) led them to opposite sides of a war. They may even have genuinely been friends, at one point. To see a man he may have genuinely liked, had known for years, had worked with closely, and certainly wished to pardon from death or even dishonor or loss of property, served to him as a severed head? That undoubtedly provoked some strong emotions.
For those reasons, Ptolemy's gift was... not well-received by Caesar.