Here is how to reproduce the malbehavior:

1. create a new spreadsheet in Calc (happened with OO3.1.0)

2. insert two rows of data, as well as to rows for error bar data, e.g.:
   x     y1     y2    erry1    erry2
   1     2      5       1        2
   2     2      5       1        2

3. mark the first three rows and create an xy plot diagram -> works fine so far.

4. now mark only the first plotted line (generated from the y1 data) in the
diagram and use the menu command "insert - Yerrorbar" and select only the data
of the column "erry1" for symmetric error bars from cell data.

5. result: these error bar values are used by OO to generate error bars on BOTH
plotted lines. (This is already wrong behavior, but could probably be tolerated.)

6. Now, explicitly select the second line in the plot (generated from the y2
data) and try to add different error bars (with the erry2 values) using the same
commands as above.

7. result: now the erry2 values are used for BOTH plots (for the y1 and for the
y2 line). At this point the frustration of the user definitly explodes! It seems
to be impossible to generate plots with two lines that have INDEPENDENT error
bar data.

To be honest, I eventually did find a workaround to still use OO Calc to
generate the expected plot with two independent error bar'ed lines (via first
doubly clicking and then right clicking on the wrongly plotted error bars
themselves and then use "object properties" and change the targeted cell area).
But this is a totally complex procedure that hardly any other user will find by
chance :(

Since the occurance of plots with error bars in combination with plots with more
then one data line is not uncommon at all in the sciences as well as in
medicine, I consider this a significant malfunction, hence a P3. 

Additionally, it is a pity. OO CAN do these plots, just some internal function
calls seem to be connected wrongly in the code. So go ahead, do the small change
and let OO show what I already can do! :)