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I have Inkscape 0.91.

Here is the behaviour when I try to select a line that lies on a horizontal guide. As the pointer approaches the guide, the pointer changes to a pointing finger, and the guide turns red. If I now click, nothing happens. The line is not selected.

If I move the pointer about 1 pixel above or below the guide, the "plus" icon appears attached to the pointer. I can now click to select the line.

The behaviour for a vertical line is similar.

I simply wish to confirm that this behaviour is correct. It took me a lot of experimenting to work it all out.

I seem to remember that with a very ancient package called "Corel Draw", the rule seemed to be that a click always selected the line. To select the guide, you had to click an empty part of the guide. In other words, the line had the priority for the focus.
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It's probably correct, that behaviour you describe.

There is two workarounds if you struggle to select a line that parrallells to a guide:

1. Click LMB before it turns to a hand. If you're close to the line (screen distance), you should still be able to select the line
2. Keep down Alt button while drawing a selection line. When holding down Alt button, everything the line touches get selected.

[edit]
The plus symbol you're talking about is not a pluss at all. It's a four directional arrow that tell you that you can move something in all directions now.