Fix the 'besiege' and 'send reinforcement' buttons for cities you are besieging.
If you are besieging a city, and want to send additional units to add to the siege (eg, because another player is also attacking that city), you cannot use the 'besiege' button beside the city... that causes your new units to attack your units already besieging, as a warning notice tells you. That is utterly stupid. There's no reason to want to attack your own besieging troops.
The only way to reinforce your besieging troops is to use the 'send reinforcements' button. The problem with this is that if your troops besieging the city are defeated (eg, by a raid) before your reinforcements get there, then when they do get there, they don't besiege the city, they become reinforcements to the city.
I find this really frustrating, and the solution is so simple: just make the 'besiege' button always besiege, and if you already have besieging troops there, have it allow you to reinforce them.