When is an avocado not safe to eat?

I'm curious too

Another site said that the "stringly" avocados (brown string-like strands inside) are from young trees. As avocados have become more popular, more trees are being planted and the stringly ones are probably from new farms or farms that planted new fields. Avocados grow on trees if you didn't know, so the trees take a long time to grow and the more mature trees produce better fruit apparantly.

I just ate one that's flesh turned a light purple in my fridge over four days. It's cousin was left on the kitchen counter, no light, average temperature around 70F and in a day and a half had large brown spots which I cut out. Both had colored strings, though they weren't fibrous, just colors. So a near mature tree? Anyway, I'll let you know if this purple one doesn't kill me. If it tries to, hopefully I'll have enough time to call 911.

If the avocado isn't too stringy, I use it for baking: chocolate avocado brownies; choco avo cookies, etc.. Just made some with a very slightly stringy avocado.

My avocado's skin color turns red, is good to eat?

Avocados are toxic to dogs.

I think Gourmet Metrics answer about the avocado being not fully mature is correct. I once inquired into this myself. Those strings are the worst texturally, and when they're present, I usually discard. But not because they're not safe.

Nutcakes - Isn't that pretty much true for eating anything before skydiving? ;) (I have a funny story about eating before skydiving, but it isn't appropriate here...)

Do not eat while skydiving.

I cut out the brown spots and eat the rest. As for the strings, I find avocados with strings in them have deteriorated in flavor and do not taste good, so I usually throw them out. They are probably safe however. As to why the strings are there in the first place, here is what the California Avocado Board had to say:

Why does my avocado have strings or spots?
Strings or stringy fruit or the thickening of the vascular bundles (fibers that run longitudinally through the fruit) are generally the result of fruit from younger trees or improper storage conditions. Often times the fibers or strings will disappear or become less noticeable as the fruit (and tree) matures.

Flesh discoloration can occur when the avocado has been exposed to cold temperatures for a long period of time. Flesh bruising can occur in transit or as a result of compression caused by excessive handling. Unfortunately there is no way to detect either flesh discoloration or flesh bruising by looking at the avocado's exterior. Damaged areas or spots can be removed by cutting them out.