Stuck notes when using computer keyboard control
You may find, when using the computer keyboard to control Cutupsampler, that some notes will "stick" — that is, will not stop when you release their keys — when you play a number of keys at a time. You will also find that Cutupsampler seems not to recognize key-presses as well when you are simultaneously using the mouse to manipulate the dials. The stuck-keys phenomenon is also more likely to happen when you are simultaneously moving the dials.
This was a problem back in 2010, which was due to OS X's key event implementation, according to a post by one of Max/MSP's programmers on the Cycling74 forum. (Cutupsampler was created with Max/MSP.) I don't know if it's still an issue in 2018.
As mentioned in the section on computer keyboard control, you can use the space bar to stop notes that are being held.
On a related note, the computer keyboard only allows for six keys to be played at once. I think that this may be due to the general design of a computer keyboard's circuitry; in any case, whether it's that or OS X, it is a minor limitation without any apparent workaround.