I have some text, it flies in from the left to the centre, then I want it to stay centre before it starts to spin.
I've done this by keyframing it off screen at 0frames, keyframing it at centre at 50frames (which works fine) but when I use the EXACT same keyframe at 60frames, the text flies past the centre and then back to centre.
take a look at the F-Curve of the animation in the Timeline. It will overshoot your value between frame 50 and 60 because the default is to have bezier curves in the timeline (they have the most control). You can select the keyframe at frame 50 (you don't need to be in the timeline you can select it with the mini-timeline in the "Standard" viewport config) and either set Interpolation to be Linear or check off Clamp