A few examples that I can recall:
The Copterline S-76 accident is an obvious example where serious side effects led to loss of control. Worth googling the accident report.
I know of a BHL S-76A+ that suffered a pin-hole fracture in the pipe that supplied the VTA. The hole was in the inside of the knee bend that takes the pipe under the centre console and the 3000 psi atomised vapour filled the cockpit with fumes. Pretty unpleasant by all accounts but fortunately no ignition source!
I know the pilot of a B212 that had a hydraulic reservoir topped up with industrial strength detergent. After a short flight (marine pilot winching) they returned and shutdown. The first symptom all was not well was that the blades could not be turned to tie them down! Prolonged flight may well have resulted in MGB failure.
A certain GoM S-92 operator had an aircraft where a hydraulic pipe was chaffed by the engine heater mat wiring. When the heater was turned on during a drying run, the arcing ignited the now leaking fluid. The subsequent 4000 psi fed conflagration was quite a spectacle I believe! The roof melted and had it been in flight would have been catastrophic (one assumes.)
I once had an 'o' ring 'let go' during a ferry flight, in a factory fresh EC-155, in Cote D'Ivoire which dumped the system out the side. The only consequence was the mess it made of the airframe! Actually the other consequence was having to spend an unplanned night stop in a s**thole! Interestingly, the part was not shown in the IPC, so we just got a big bag of assorted 'o' rings sent. Once fixed we continued to Accra and got an MGB Chip enroute - great these new machines!