Hidden Line Bug

I’ve had this problem many times over a few years but I don’t know how to avoid it. When I take a certain random action (it’s different each time) suddenly this will happen on a face near to the one I’m working on. This time it happened due to deleting a line on a face that wasn’t even adjacent. The lines appear hidden at first, but upon selecting ‘unhide all’ the lines appear, which have broken up the face into many smaller triangular faces. These smaller faces obviously cannot be treated as one, so I’m often faced with the pain-staking task of deleting each new line one at a time.

Does anyone know why this is happening and how I can stop it?

The usual reason behind this is the face is ever so slightly not flat, flat enough that SU sees it as co-planer until it doesn’t. Changing one edge can make the tiny distortion in the face shatter.
This is most often caused by tiny errors introduced by using length snapping, although not always the reason.
Go to Model info/Units and Disable ‘Enable Length Snapping’. If you already have that unticked then you need to explore other possibilities.

@devanreid, maybe you could share the file here?