Hi,
As audio guru pointed out, the transistor can easily get damaged with an out of spec reverse voltage.
This is interesting because what happens is you might not even notice the damage to the transistor after a brief reverse Vbe that is higher than its data sheet rating. The effect can be noticed sometimes in the decrease in gain but the transistor still appears to work normally with a casual test. It's only when you go to try to get the transistor working in a circuit that requires the full spec's of the transistor (like gain and frequency response) that you can notice the difference.
For example, using a transistor in an RF oscillator circuit it might work just fine, but after a brief reverse Vbe voltage a little higher than the data sheet spec and then try to use that transistor in that same RF oscillator circuit and it probably wont work. This only takes a brief reverse voltage too, maybe less than a quarter second or even less.
So what happens is the transistor does get damaged but it doesnt look like sometimes it until you give it a full test of its capabilities. This happens with LEDs too sometimes. They still work for a while after a higher than normal reverse voltage but then one day you go to turn it on and it doesnt work at all, not even dim but completely dead.
Unfortunately the reverse Vbe can happen so easily, just by mixing up the transistor base and emitter leads and connecting it to a circuit. Suddenly it sees more than 5v reverse voltage across the base and emitter. Then after realizing the 'small' problem and fixing the leads, the transistor doesnt work as well anymore and has to be completely replaced.
This is also one of the reasons that in higher power drive circuits the drive transistors are changed when the main power transistor blows out even though the drive circuit transistors dont appear to be damaged. The damage isnt immediately apparent.