Destination Array not long enough?

I would say the error lies in the object _bikes not being thread safe. As commented, somewhere there is a modify of the _bikes object that is not being lock'ed.

It is a split second error where the variable bs is set up to a size X when the size of _bikes is measured. In the next split second as it is about to fill the list, the _bikes object has increased in size giving the error.

So go over your code. Find all references of your _bikes object and make sure they are thread safe handled (with lock).

This error occurs because multiple threads are adding items in a single list. Lists are by default not a thread-safe solution. In a multi-threaded code, it is only recommended to read from a list, and not write to it.

As described here:

If you are only reading from a shared collection, then you can use the classes in the System.Collections.Generic namespace.

Better use a thread-safe solution like System.Collections.Concurrent namespace which provides implementations like ConcurrentBag, ConcurrentDictionary, ConcurrentQueue, ConcurrentStack etc.

For example:

public ConcurrentBag<Bike> bikesCopy 
{
     get => new ConcurrentBag<Bike>()
}

Well you could try:

using System.Linq; //ToList() is an extension function defined here
...
lock(_bikes)
    return _bikes.ToList();

The details of the exception are discussed here: Why doesn't a foreach loop work in certain cases?