Squint surgery is a safe procedure, and complications are rare. And, in any case, they can be treated effectively. Complications may include over or under-correction, double vision, or infections.
If the defect is over or under corrected, you can have another surgery. This complication may occur because the stitch(es) get loosen.
Double vision (diplopia) is common after surgery. On rare occasions, it can be more persistent or permanent. Lastly, loss of sight after a squint operation is extremely rare, happening in 1 in 20,000 cases.