Steps:
1. Select the eraser tool in the mode "Cut out from object."
2. Left-click to activate the tool and maintain the left-button clicked while selecting the region to erase
3. While still maintaining the left-button clicked, press on the right button. The contextual menu pops up.
4. Get the mouse pointer out of the contextual menu to somewhere else in the drawing
Alternative:
5a (non buggy, cool feature). Release the right button (the left button is still pressed). At this point, the eraser tool will consider the last point before the right-click, the first point after the right click and fill in the eraser too path with a calligraphic stroke, and will allow the user to go on selecting the eraser path at its normal width. When the left button is finally released, all this path is erased.
5b-1 (buggy). Release the left button (the right button is still pressed). Then, an indicative (almost meaningless) thin line is drawn along the movements of the mouse pointer. See picture-1
5b-2 When the user clicks on the left button again, the tool restarts choosing the eraser path (the meaningless line is cleared). Visually, the impression is that it allows to select an interrupted eraser path. See picture-2.
5b-3 When the left button is released, the path between the two "release/press" points is erased according to a calligraphic stroke which the user never could see. See picture-3
Compare in particular the picture-2 with the clearly interrupted eraser path and the picture-3 which shows, at the place the path was interrupted, the calligraphic stroke which is applied without being presented to the user sooner.
Expected result:
At step 5b-2, the when the user clicks a second time on the left button, the indicative (meaningless) line should be replaced by the calligraphic path that is anyway going to be applied.
Linux x86_64, gcc 4.3.2, inkscape r21126 (snapshot 12 April 2009)
Similar oddities are present in other tools, including the freehand tool. In my opinion, right-clicking simply shouldn't do anything if the left button is down.
The bezier tool works a little differently than the others, but still doesn't seem entirely predictable: if the left button is not down, clicking the right button places a final node. If the left button is down (e.g. you are setting curvature), clicking the right button only places the handle(?) and ends the path, but doesn't make an additional node. I also think this tool should do nothing if the left button is down and the right button gets clicked.
See also bugs #169207 and #208768.