Not a common problem as I have the same issue on my R53 where a pin hole (emphasis on pinhole) was created when the throwout bearing shattered. The bolt most likely at some stage was damaged on a prior service or may have been faulty. Your up for a rebuilt gearbox and you wont get any money off as a core exchange due to the damage to the casing. I have just spent $2k on a SH Gearbox which I intend to swap out soon as I fear that my current leak may be from the alloy weld repair that may have failed. Alloy weld repairs are hit or miss due to hidden stress cracks that may open up when welding or afterwards and are very difficult to prevent. You will need to check for oil contamination on the clutch plate and while your at it, wear to the flywheel and any swarf/debris that may have entered. The good news is, its the perfect time to check and replace the rear main seal to the engine and if your clutch/flywheel is contaminated or damaged, a low cost option is the Valeo single mass flywheel and clutch kit (cheaper than just the OEM flywheel alone). Modmini on YouTube has a video on the Valeo unit and also a good DIY on swapping it all out.
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