I realize this is rather old hardware, at least as digital goes, but I'm hoping to hand it down to a friend as a starter kit. Every once in a while, my Digital Rebel XS will refuse to take a shot through the 55-250 - the switch on the lens is set to MF, the shot is clear, but all that happens is the right-most green light in the viewfinder starts blinking. Oddly, switching back to AF works - lets the shot be taken - then switching to MF it fails again. No on-screen message, just the blinking of what the user guide calls the "Focus Confirmation Light".
In the past, taking the lens off and putting it back on has cleared the problem, but not today; this time the only thing that worked was trying it on a T2i (trying to test camera vs. lens) where both AF and MF worked fine, then putting it back on the Rebel (which I hope was coincidence...)
Any ideas as to what the root cause might be? Cleaning the lens contacts didn't seem to make a difference (and as I mention above, AF mode worked fine.)